Extra none:none images after docker build
Those images are the docker build cache. You need this to be able to quickly rebuild similar images, and to avoid rebuilding unchanged layers which would result in resending those to remote registry servers and forcing servers to download those unchanged layers again. One important point on these layers:

They do not consume any significant disk space

You can view the layers of an image by running the docker image history command on an image. Here's an example of a rather large image built on my machine:

$ docker image history 25f90c8ba1ca
IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
25f90c8ba1ca        11 days ago         /bin/sh -c #(nop)  CMD ["/bin/sh" "-c" "echo…   0B
8a190467427e        11 days ago         /bin/sh -c #(nop) COPY dir:36bb9e67a0f2844c2…   298MB
1f6f5474071d        11 days ago         /bin/sh -c #(nop) COPY dir:7c364877951bb2312…   249MB
007510a95d17        11 days ago         /bin/sh -c #(nop) COPY dir:9817b6e3d212385d0…   231MB
ee23b7a42488        11 days ago         /bin/sh -c #(nop) COPY dir:57e817f3d9d977f66…   277MB
e190d1bb3f66        11 days ago         /bin/sh -c #(nop) COPY dir:9ba997e629e74ca64…   205MB
abe0ec5c0ef9        11 days ago         /bin/sh -c #(nop) COPY dir:4dbbd5b2010590e28…   213MB
544af937d8a3        11 days ago         /bin/sh -c #(nop) COPY dir:0ac40edba2c7e1317…   178MB
b78085aba431        11 days ago         /bin/sh -c #(nop) COPY dir:2137a075c552d6783…   135MB
3ff8488b067b        11 days ago         /bin/sh -c #(nop) COPY dir:45da2005488e81e9c…   112MB
19b7cdd9d682        11 days ago         /bin/sh -c #(nop) COPY dir:2b745a4311740ddf7…   65.5MB
eb2cf84859fb        11 days ago         /bin/sh -c #(nop) COPY dir:cd7b639f2eee0e9da…   50.1MB
1ba1c21bb0a3        11 days ago         /bin/sh -c #(nop) COPY dir:594739607d1f2d593…   51.9MB
1da5d6bf03e9        11 days ago         /bin/sh -c #(nop) COPY dir:757daf68a395cfaae…   30MB
f6df8b695822        11 days ago         /bin/sh -c #(nop) COPY dir:449b6c2ff93606741…   18.5MB
72895d213dac        11 days ago         /bin/sh -c #(nop) COPY dir:2290eb8d11f15dbe6…   13.8MB
3b6ceadce77b        11 days ago         /bin/sh -c #(nop) COPY file:fcd71a6bfc87e9dd…   167B
52dd0e701be3        11 days ago         /bin/sh -c #(nop)  ENV TARGET=/usr/share/ngi…   0B
aae476eee77d        3 weeks ago         /bin/sh -c #(nop)  CMD ["nginx" "-g" "daemon…   0B
&lt;missing&gt;           3 weeks ago         /bin/sh -c #(nop)  STOPSIGNAL [SIGTERM]         0B
&lt;missing&gt;           3 weeks ago         /bin/sh -c #(nop)  EXPOSE 80/tcp                0B
&lt;missing&gt;           3 weeks ago         /bin/sh -c #(nop) COPY file:1d1ac3b9a14c94a7…   1.09kB
&lt;missing&gt;           3 weeks ago         /bin/sh -c #(nop) COPY file:af94db45bb7e4b8f…   643B
&lt;missing&gt;           3 weeks ago         /bin/sh -c GPG_KEYS=B0F4253373F8F6F510D42178…   13.3MB
&lt;missing&gt;           3 weeks ago         /bin/sh -c #(nop)  ENV NGINX_VERSION=1.15.5     0B
&lt;missing&gt;           4 weeks ago         /bin/sh -c #(nop)  LABEL maintainer=NGINX Do…   0B
&lt;missing&gt;           6 weeks ago         /bin/sh -c #(nop)  CMD ["/bin/sh"]              0B
&lt;missing&gt;           6 weeks ago         /bin/sh -c #(nop) ADD file:25c10b1d1b41d46a1…   4.41MB

$ docker image inspect --format '{{json .RootFS.Layers}}' 25f90c8ba1ca | jq
.
[
  "sha256:df64d3292fd6194b7865d7326af5255db6d81e9df29f48adde61a918fbd8c332",
  "sha256:39e8483b9882aa8e95fb00c14e1c5dbdf32e836af4aff4bf862608e0a35bc914",
  "sha256:431a5c7929dd5848b9734d84d5cc5be237dd84e9e5ab1cb84ad6975bdc171c0c",
  "sha256:a83dbde6ba05e8112bd2732375ffbd350d9872d9c9e7e088ba5127118cd6d99e",
  "sha256:65ec7af97662fd3f933dc8b0aefb9f2d9d25c0d78530142bcdab1323c6aee223",
  "sha256:25212046c4c3a7589e45b426085da35d7e5e5a7a834180977a43df533672c360",
  "sha256:508c1d799f8da8accdd8b4e92964c73d6f9b8c4e7381fd0e1241e894d01418d3",
  "sha256:a197e87da4cd4587c8965d81878fbb1d898c8ba7d2a05ff0e9862eb700d26053",
  "sha256:f66588501183b7000c7683808ec4653bb83dc3c07f77d67fc34c78e9d9e5cb33",
  "sha256:de2288e4aa944daa6059184813c549d994f5c2407bfae61c7f8b4e0f950e73b1",
  "sha256:77ae40a90a8d41d53751ee800d4008d7a3706ecae6364560b4606ba0167c2674",
  "sha256:1a4bae64e86919b1f40e61a6022e8f910748566b88c2628893c4e424a60c74a2",
  "sha256:ca61f354107fc14de3c01f5bb192ff8b673b6f84c7089a23d754b23a65751310",
  "sha256:d0da652f2c59d6fb6380c0b6940aaa9f5e5a4fc28ae0b82f1acb5ccf9a1884e1",
  "sha256:5f3a91149be048f4f0a4d2450eab6ff8e630bb416b5c508633014e645f57df06",
  "sha256:0c0762559bcfec46462749615ee06b0998e61673e8118cd1a43e20b38e27825a",
  "sha256:581ad1603f59463febb540052046bbdec82bdfc7c75fd4a94578bdca88b91fec",
  "sha256:ff599edf8583ba1dfd7d9c79c1b718f94641e1fb644a0be29ca5b45dcc1768b4",
  "sha256:6ce4a45f89645a2020cfdae51c83470d9f24424ea16f4bddaf1244aa08f2945e",
  "sha256:5fcfba1ba609a8fbfc3eccbc805eb18a78041897ec3f6dc168c9ed871baed950"
]

$ docker image inspect --format '{{json .RootFS.Layers}}' 8a190467427e | jq .
[
  "sha256:df64d3292fd6194b7865d7326af5255db6d81e9df29f48adde61a918fbd8c332",
  "sha256:39e8483b9882aa8e95fb00c14e1c5dbdf32e836af4aff4bf862608e0a35bc914",
  "sha256:431a5c7929dd5848b9734d84d5cc5be237dd84e9e5ab1cb84ad6975bdc171c0c",
  "sha256:a83dbde6ba05e8112bd2732375ffbd350d9872d9c9e7e088ba5127118cd6d99e",
  "sha256:65ec7af97662fd3f933dc8b0aefb9f2d9d25c0d78530142bcdab1323c6aee223",
  "sha256:25212046c4c3a7589e45b426085da35d7e5e5a7a834180977a43df533672c360",
  "sha256:508c1d799f8da8accdd8b4e92964c73d6f9b8c4e7381fd0e1241e894d01418d3",
  "sha256:a197e87da4cd4587c8965d81878fbb1d898c8ba7d2a05ff0e9862eb700d26053",
  "sha256:f66588501183b7000c7683808ec4653bb83dc3c07f77d67fc34c78e9d9e5cb33",
  "sha256:de2288e4aa944daa6059184813c549d994f5c2407bfae61c7f8b4e0f950e73b1",
  "sha256:77ae40a90a8d41d53751ee800d4008d7a3706ecae6364560b4606ba0167c2674",
  "sha256:1a4bae64e86919b1f40e61a6022e8f910748566b88c2628893c4e424a60c74a2",
  "sha256:ca61f354107fc14de3c01f5bb192ff8b673b6f84c7089a23d754b23a65751310",
  "sha256:d0da652f2c59d6fb6380c0b6940aaa9f5e5a4fc28ae0b82f1acb5ccf9a1884e1",
  "sha256:5f3a91149be048f4f0a4d2450eab6ff8e630bb416b5c508633014e645f57df06",
  "sha256:0c0762559bcfec46462749615ee06b0998e61673e8118cd1a43e20b38e27825a",
  "sha256:581ad1603f59463febb540052046bbdec82bdfc7c75fd4a94578bdca88b91fec",
  "sha256:ff599edf8583ba1dfd7d9c79c1b718f94641e1fb644a0be29ca5b45dcc1768b4",
  "sha256:6ce4a45f89645a2020cfdae51c83470d9f24424ea16f4bddaf1244aa08f2945e",
  "sha256:5fcfba1ba609a8fbfc3eccbc805eb18a78041897ec3f6dc168c9ed871baed950"
]

$ docker image inspect --format '{{json .RootFS.Layers}}' 1f6f5474071d | jq .
[
  "sha256:df64d3292fd6194b7865d7326af5255db6d81e9df29f48adde61a918fbd8c332",
  "sha256:39e8483b9882aa8e95fb00c14e1c5dbdf32e836af4aff4bf862608e0a35bc914",
  "sha256:431a5c7929dd5848b9734d84d5cc5be237dd84e9e5ab1cb84ad6975bdc171c0c",
  "sha256:a83dbde6ba05e8112bd2732375ffbd350d9872d9c9e7e088ba5127118cd6d99e",
  "sha256:65ec7af97662fd3f933dc8b0aefb9f2d9d25c0d78530142bcdab1323c6aee223",
  "sha256:25212046c4c3a7589e45b426085da35d7e5e5a7a834180977a43df533672c360",
  "sha256:508c1d799f8da8accdd8b4e92964c73d6f9b8c4e7381fd0e1241e894d01418d3",
  "sha256:a197e87da4cd4587c8965d81878fbb1d898c8ba7d2a05ff0e9862eb700d26053",
  "sha256:f66588501183b7000c7683808ec4653bb83dc3c07f77d67fc34c78e9d9e5cb33",
  "sha256:de2288e4aa944daa6059184813c549d994f5c2407bfae61c7f8b4e0f950e73b1",
  "sha256:77ae40a90a8d41d53751ee800d4008d7a3706ecae6364560b4606ba0167c2674",
  "sha256:1a4bae64e86919b1f40e61a6022e8f910748566b88c2628893c4e424a60c74a2",
  "sha256:ca61f354107fc14de3c01f5bb192ff8b673b6f84c7089a23d754b23a65751310",
  "sha256:d0da652f2c59d6fb6380c0b6940aaa9f5e5a4fc28ae0b82f1acb5ccf9a1884e1",
  "sha256:5f3a91149be048f4f0a4d2450eab6ff8e630bb416b5c508633014e645f57df06",
  "sha256:0c0762559bcfec46462749615ee06b0998e61673e8118cd1a43e20b38e27825a",
  "sha256:581ad1603f59463febb540052046bbdec82bdfc7c75fd4a94578bdca88b91fec",
  "sha256:ff599edf8583ba1dfd7d9c79c1b718f94641e1fb644a0be29ca5b45dcc1768b4",
  "sha256:6ce4a45f89645a2020cfdae51c83470d9f24424ea16f4bddaf1244aa08f2945e"
]


In each of those inspect commands you should see that each new image is just adding on an extra layer until you reach the final image. These layers are the same sha256 checksums, pointing to the same files on the disk. Docker does not copy these files per image, it doesn't even copy them when creating containers (excluding initializing named volumes), the layers are read-only and reused by every descendant image and container via a union filesystem. The disk usage that docker displays is the sum of all of the layers even when other images may share layers, while your actual disk usage is only of the unique layers.

If you rebuild images often on the server with the same build tag, you can prune dangling images with docker image prune. Otherwise, simply avoid displaying these build artifacts by running docker image ls without the -a option.