How to pin public root key when downloading an image with docker pull (Docker Content Trust)?
tl;dr
You can execute the following to pin the public root key for debian:
sudo su -
mkdir -p /root/.docker/trust/tuf/docker.io/library/debian/metadata
chown -R root:root /root/.docker
chmod -R 0700 /root/.docker
echo '{&quot;signed&quot;:{&quot;_type&quot;:&quot;Root&quot;,&quot;consistent_snapshot&quot;:false,&quot;expires&quot;:&quot;2025-08-07T20:55:22.677722315-07:00&quot;,&quot;keys&quot;:{&quot;5717dcd81d9fb5b73aa15f2d887a6a0de543829ab9b2d411acce9219c2f8ba3a&quot;:{&quot;keytype&quot;:&quot;ecdsa&quot;,&quot;keyval&quot;:{&quot;private&quot;:null,&quot;public&quot;:&quot;MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEsslGF2xHOYztrocb2OsRF2zth16v170QiLAyKdce1nQgOJ34FOk679ClPL9/RNnJukf2JfQXSlVV/qcsvxV2dQ==&quot;}},&quot;575d013f89e3cbbb19e0fb06aa33566c22718318e0c9ffb1ab5cc4291e07bf84&quot;:{&quot;keytype&quot;:&quot;ecdsa-x509&quot;,&quot;keyval&quot;:{&quot;private&quot;:null,&quot;public&quot;:&quot;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&quot;}},&quot;728c96ff5e9f48d4e66d5a0c3ecabfdd90bee2b5f9f80b950ed9c668db264a70&quot;:{&quot;keytype&quot;:&quot;ecdsa&quot;,&quot;keyval&quot;:{&quot;private&quot;:null,&quot;public&quot;:&quot;MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAENtpBkDJ2oYaAAVdOkP0A6J0XwUkYGuFRk+q8N4WCPu2VnNIuBJkatPCWdEtHfQ9nNYLeanWgG62/UmJnx3E2Yg==&quot;}},&quot;d48327d85f0490827db7c931eedb58d293e1da5fc425ea0cde3e6c13b397ad69&quot;:{&quot;keytype&quot;:&quot;ecdsa&quot;,&quot;keyval&quot;:{&quot;private&quot;:null,&quot;public&quot;:&quot;MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEwfs26T/cpjvNTXVJpK7Wv8oDOnNKL78AT3Y1QD356OIAggwPupX2LQjZU6CVzCjm+pkJIO4clu9Q2n540gKuzQ==&quot;}}},&quot;roles&quot;:{&quot;root&quot;:{&quot;keyids&quot;:[&quot;575d013f89e3cbbb19e0fb06aa33566c22718318e0c9ffb1ab5cc4291e07bf84&quot;],&quot;threshold&quot;:1},&quot;snapshot&quot;:{&quot;keyids&quot;:[&quot;d48327d85f0490827db7c931eedb58d293e1da5fc425ea0cde3e6c13b397ad69&quot;],&quot;threshold&quot;:1},&quot;targets&quot;:{&quot;keyids&quot;:[&quot;5717dcd81d9fb5b73aa15f2d887a6a0de543829ab9b2d411acce9219c2f8ba3a&quot;],&quot;threshold&quot;:1},&quot;timestamp&quot;:{&quot;keyids&quot;:[&quot;728c96ff5e9f48d4e66d5a0c3ecabfdd90bee2b5f9f80b950ed9c668db264a70&quot;],&quot;threshold&quot;:1}},&quot;version&quot;:1},&quot;signatures&quot;:[{&quot;keyid&quot;:&quot;575d013f89e3cbbb19e0fb06aa33566c22718318e0c9f
fb1ab5cc4291e07bf84&quot;,&quot;method&quot;:&quot;ecdsa&quot;,&quot;sig&quot;:&quot;3WbX1VXN9E8LRmSG+E4SQlBUNqBNchhwAStWnRWLLyAOoFNBq5xmIgSO3UYYuKyJvL7kbMoONRbn5Vk2p2Wqrg==&quot;}]}' &gt; /root/.docker/trust/tuf/docker.io/library/debian/metadata/root.json

export DOCKER_CONTENT_TRUST=1
docker pull debian:stable-slim

Long Answer

⚠ Disclaimer: I am not a docker developer. As of 2021, it appears that DCT is broken out-of-the-box and is far from being useful. My answer here is a best-guess, but I have not confirmed with the docker team if this is the &quot;correct&quot; way to pre-load and pin a given publisher's root key into the DCT keyring.
Be advised, proceed with caution, and your comments are very welcome.

It doesn't appear to be documented anywhere, but per this question it's clear that docker puts its DCT metadata (including root public keys) in the following location:
$HOME/.docker/trust/tuf/docker.io/library

Inside this library dir exists one dir per publisher. For the purposes of this answer, I'll use debian as our example publisher.
You can see the list of the debian docker images published to Docker Hub here:

https://hub.docker.com/_/debian/

Solution
Let's say we want to download the stable-slim image from the debian publisher on Docker Hub. In this example, we'll also use a fresh install of Debian 10 as the docker host.
##
# first, install docker
## 
root@disp2716:~# apt-get install docker.io
...
root@disp2716:~#

##
# confirm that there is no docker config dir yet
##

root@disp2716:~# ls -lah /root/.docker
ls: cannot access '/root/.docker': No such file or directory
root@disp2716:~# 

##
# add the debian publisher's root DCT key
##

root@disp2716:~# mkdir -p /root/.docker/trust/tuf/docker.io/library/debian/metadata
root@disp2716:~# chown -R root:root /root/.docker
root@disp2716:~# chmod -R 0700 /root/.docker
root@disp2716:~# echo '{&quot;signed&quot;:{&quot;_type&quot;:&quot;Root&quot;,&quot;consistent_snapshot&quot;:false,&quot;expires&quot;:&quot;2025-08-07T20:55:22.677722315-07:00&quot;,&quot;keys&quot;:{&quot;5717dcd81d9fb5b73aa15f2d887a6a0de543829ab9b2d411acce9219c2f8ba3a&quot;:{&quot;keytype&quot;:&quot;ecdsa&quot;,&quot;keyval&quot;:{&quot;private&quot;:null,&quot;public&quot;:&quot;MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEsslGF2xHOYztrocb2OsRF2zth16v170QiLAyKdce1nQgOJ34FOk679ClPL9/RNnJukf2JfQXSlVV/qcsvxV2dQ==&quot;}},&quot;575d013f89e3cbbb19e0fb06aa33566c22718318e0c9ffb1ab5cc4291e07bf84&quot;:{&quot;keytype&quot;:&quot;ecdsa-x509&quot;,&quot;keyval&quot;:{&quot;private&quot;:null,&quot;public&quot;:&quot;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&quot;}},&quot;728c96ff5e9f48d4e66d5a0c3ecabfdd90bee2b5f9f80b950ed9c668db264a70&quot;:{&quot;keytype&quot;:&quot;ecdsa&quot;,&quot;keyval&quot;:{&quot;private&quot;:null,&quot;public&quot;:&quot;MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAENtpBkDJ2oYaAAVdOkP0A6J0XwUkYGuFRk+q8N4WCPu2VnNIuBJkatPCWdEtHfQ9nNYLeanWgG62/UmJnx3E2Yg==&quot;}},&quot;d48327d85f0490827db7c931eedb58d293e1da5fc425ea0cde3e6c13b397ad69&quot;:{&quot;keytype&quot;:&quot;ecdsa&quot;,&quot;keyval&quot;:{&quot;private&quot;:null,&quot;public&quot;:&quot;MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEwfs26T/cpjvNTXVJpK7Wv8oDOnNKL78AT3Y1QD356OIAggwPupX2LQjZU6CVzCjm+pkJIO4clu9Q2n540gKuzQ==&quot;}}},&quot;roles&quot;:{&quot;root&quot;:{&quot;keyids&quot;:[&quot;575d013f89e3cbbb19e0fb06aa33566c22718318e0c9ffb1ab5cc4291e07bf84&quot;],&quot;threshold&quot;:1},&quot;snapshot&quot;:{&quot;keyids&quot;:[&quot;d48327d85f0490827db7c931eedb58d293e1da5fc425ea0cde3e6c13b397ad69&quot;],&quot;threshold&quot;:1},&quot;targets&quot;:{&quot;keyids&quot;:[&quot;5717dcd81d9fb5b73aa15f2d887a6a0de543829ab9b2d411acce9219c2f8ba3a&quot;],&quot;threshold&quot;:1},&quot;timestamp&quot;:{&quot;keyids&quot;:[&quot;728c96ff5e9f48d4e66d5a0c3ecabfdd90bee2b5f9f80b950ed9c668db264a70&quot;],&quot;threshold&quot;:1}},&quot;version&quot;:1},&quot;signatures&quot;:[{&quot;keyid&quot;:&quot;575d013f89e3cbbb19e0fb06aa33566c22718318e0c9ffb1ab5cc4291e07bf84&quot;,&quot;method&quot;:&quot;ecdsa&quot;,&quot;sig&quot;:&quot;3WbX1VXN9E8LRmSG+E4SQlBUNqBNchhwAStWnRWLLyAOoFNBq5xmIgSO3UYYuKyJvL7kbMoONRbn5Vk2p2Wqrg==&quot;}]}' &gt; /root/.docker/trust/tuf/docker.io/library/debian/metadata/root.json
root@disp2716:~# 
root@disp2716:~# chown root:root /root/.docker/trust/tuf/docker.io/library/debian/metadata/root.json
root@disp2716:~# chmod 0600 /root/.docker/trust/tuf/docker.io/library/debian/metadata/root.json
root@disp2716:~# 

##
# pull the docker image with DCT verification
##

root@disp2716:~# export DOCKER_CONTENT_TRUST=1
root@disp2716:~# docker pull debian:stable-slim
Pull (1 of 1): debian:stable-slim@sha256:850a7ee21c49c99b0e5e06df21f898a0e64335ae84eb37d6f71abc1bf28f5632
sha256:850a7ee21c49c99b0e5e06df21f898a0e64335ae84eb37d6f71abc1bf28f5632: Pulling from library/debian
6e640006d1cd: Pull complete 
Digest: sha256:850a7ee21c49c99b0e5e06df21f898a0e64335ae84eb37d6f71abc1bf28f5632
Status: Downloaded newer image for debian@sha256:850a7ee21c49c99b0e5e06df21f898a0e64335ae84eb37d6f71abc1bf28f5632
Tagging debian@sha256:850a7ee21c49c99b0e5e06df21f898a0e64335ae84eb37d6f71abc1bf28f5632 as debian:stable-slim
root@disp2716:~# 

Proof
While there's no way to tell docker to fail on TOFU, we can confirm that the above key pinning works by making the public key something else
##
# first, move the docker config dir out of the way
## 

mv /root/.docker /root/.docker.bak

##
# add the debian publisher's root DCT key (note I just overwrote the first 8
# characters of the actual key with &quot;INVALID/&quot;)
##

root@disp2716:~# mkdir -p /root/.docker/trust/tuf/docker.io/library/debian/metadata
root@disp2716:~# chown -R root:root /root/.docker
root@disp2716:~# chmod -R 0700 /root/.docker
root@disp2716:~# echo '{&quot;signed&quot;:{&quot;_type&quot;:&quot;Root&quot;,&quot;consistent_snapshot&quot;:false,&quot;expires&quot;:&quot;2025-08-07T20:55:22.677722315-07:00&quot;,&quot;keys&quot;:{&quot;5717dcd81d9fb5b73aa15f2d887a6a0de543829ab9b2d411acce9219c2f8ba3a&quot;:{&quot;keytype&quot;:&quot;ecdsa&quot;,&quot;keyval&quot;:{&quot;private&quot;:null,&quot;public&quot;:&quot;INVALID/KoZIzj0CAQYIKoZIzj0DAQcDQgAEsslGF2xHOYztrocb2OsRF2zth16v170QiLAyKdce1nQgOJ34FOk679ClPL9/RNnJukf2JfQXSlVV/qcsvxV2dQ==&quot;}},&quot;575d013f89e3cbbb19e0fb06aa33566c22718318e0c9ffb1ab5cc4291e07bf84&quot;:{&quot;keytype&quot;:&quot;ecdsa-x509&quot;,&quot;keyval&quot;:{&quot;private&quot;:null,&quot;public&quot;:&quot;INVALID/RUdJTiBDRVJUSUZJQ0FURS0tLS0tCk1JSUJlVENDQVIrZ0F3SUJBZ0lRWExkUFFHTGJaOE84UXFlTzVuZlBRekFLQmdncWhrak9QUVFEQWpBak1TRXcKSHdZRFZRUURFeGhrYjJOclpYSXVhVzh2YkdsaWNtRnllUzlrWldKcFlXNHdIaGNOTVRVd09ERXhNRE0xTlRJeQpXaGNOTWpVd09EQTRNRE0xTlRJeVdqQWpNU0V3SHdZRFZRUURFeGhrYjJOclpYSXVhVzh2YkdsaWNtRnllUzlrClpXSnBZVzR3V1RBVEJnY3Foa2pPUFFJQkJnZ3Foa2pPUFFNQkJ3TkNBQVE1ZGkxcmxPQjBMQmRNS2N0VFQxYmwKUGd6aXYxOUJDdW9tNEFNL3BUdURtdjBnS0E5S1ptNUVjLy9VQmhSODVCYmR0cTk0cXhQM3IwUjhRc3FQV1Y4SQpvelV3TXpBT0JnTlZIUThCQWY4RUJBTUNBS0F3RXdZRFZSMGxCQXd3Q2dZSUt3WUJCUVVIQXdNd0RBWURWUjBUCkFRSC9CQUl3QURBS0JnZ3Foa2pPUFFRREFnTklBREJGQWlBOUFOZ3dPN2tBdUVIK3U2N25XNlFLWmlMdWd5UVcKaEQ3Vys5WjIza01mTndJaEFJa3RTaW1TdFdRQkFoOG9WOXhjaWNVWWVUN0pyUG82a0RqeHU1YitGZ3MxCi0tLS0tRU5EIENFUlRJRklDQVRFLS0tLS0K&quot;}},&quot;728c96ff5e9f48d4e66d5a0c3ecabfdd90bee2b5f9f80b950ed9c668db264a70&quot;:{&quot;keytype&quot;:&quot;ecdsa&quot;,&quot;keyval&quot;:{&quot;private&quot;:null,&quot;public&quot;:&quot;INVALID/KoZIzj0CAQYIKoZIzj0DAQcDQgAENtpBkDJ2oYaAAVdOkP0A6J0XwUkYGuFRk+q8N4WCPu2VnNIuBJkatPCWdEtHfQ9nNYLeanWgG62/UmJnx3E2Yg==&quot;}},&quot;d48327d85f0490827db7c931eedb58d293e1da5fc425ea0cde3e6c13b397ad69&quot;:{&quot;keytype&quot;:&quot;ecdsa&quot;,&quot;keyval&quot;:{&quot;private&quot;:null,&quot;public&quot;:&quot;INVALID/KoZIzj0CAQYIKoZIzj0DAQcDQgAEwfs26T/cpjvNTXVJpK7Wv8oDOnNKL78AT3Y1QD356OIAggwPupX2LQjZU6CVzCjm+pkJIO4clu9Q2n540gKuzQ==&quot;}}},&quot;roles&quot;:{&quot;root&quot;:{&quot;keyids&quot;:[&quot;575d013f89e3cbbb19e0fb06aa33566c22718318e0c9ffb1ab5cc4291e07bf84&quot;],&quot;threshold&quot;:1},&quot;snapshot&quot;:{&quot;keyids&quot;:[&quot;d48327d85f0490827db7c931eedb58d293e1da5fc425ea0cde3e6c13b397ad69&quot;],&quot;threshold&quot;:1},&quot;targets&quot;:{&quot;keyids&quot;:[&quot;5717dcd81d9fb5b73aa15f2d887a6a0de543829ab9b2d411acce9219c2f8ba3a&quot;],&quot;threshold&quot;:1},&quot;timestamp&quot;:{&quot;keyids&quot;:[&quot;728c96ff5e9f48d4e66d5a0c3ecabfdd90bee2b5f9f80b950ed9c668db264a70&quot;],&quot;threshold&quot;:1}},&quot;version&quot;:1},&quot;signatures&quot;:[{&quot;keyid&quot;:&quot;575d013f89e3cbbb19e0fb06aa33566c22718318e0c9ffb1ab5cc4291e07bf84&quot;,&quot;method&quot;:&quot;ecdsa&quot;,&quot;sig&quot;:&quot;3WbX1VXN9E8LRmSG+E4SQlBUNqBNchhwAStWnRWLLyAOoFNBq5xmIgSO3UYYuKyJvL7kbMoONRbn5Vk2p2Wqrg==&quot;}]}' &gt; /root/.docker/trust/tuf/docker.io/library/debian/metadata/root.json
root@disp2716:~# 
root@disp2716:~# chown root:root /root/.docker/trust/tuf/docker.io/library/debian/metadata/root.json
root@disp2716:~# chmod 0600 /root/.docker/trust/tuf/docker.io/library/debian/metadata/root.json
root@disp2716:~# 

##
# pull the docker image with DCT verification
##

root@disp2716:~# export DOCKER_CONTENT_TRUST=1
root@disp2716:~# docker pull debian:stable-slim
could not validate the path to a trusted root: unable to retrieve valid leaf certificates
root@disp2716:~# 
root@disp2716:~# echo $?
1
root@disp2716:~# 

Note that docker exits 1 with an error refusing to pull the debian:stable-slim docker image from Docker Hub because it cannot trust its signature