fix(README): make docker instructions clearer
Fixes
Turns out that BuildKit is required, not "strongly suggested".
@@ -33,16 +33,17 @@ compiler.
### Build a Docker Image
-To build a Docker image in one command:
-```
-docker build .
-```
-
-To enable caching, and to respect `Dockerfile.dockerignore`, we strongly suggest that you [enable BuildKit](https://docs.docker.com/develop/develop-images/build_enhancements/#to-enable-buildkit-builds).
+**BuildKit is required.**
+[Enable BuildKit](https://docs.docker.com/develop/develop-images/build_enhancements/#to-enable-buildkit-builds):
- Use Docker version 18.09 or later
-- Enable BuildKit by default by setting `{ "features": { "buildkit": true } }`
+- Enable BuildKit by default by setting `{ "features": { "buildkit": true } }` in the Docker engine config
- ...or run `docker build .` with env var `DOCKER_BUILDKIT=1`
+To build the Docker image:
+```
+DOCKER_BUILDKIT=1 docker build .
+```
+
### Cloning the Repository
Using `git`, check out the code by cloning this repository. If you use the `git` command line, this
