doc: Recommend lint image build on every call

Author: MarcoFalke

test/lint/README.md

@@ -7,13 +7,11 @@ To run linters locally with the same versions as the CI environment, use the inc
 Dockerfile:
 
 ```sh
-DOCKER_BUILDKIT=1 docker build -t bitcoin-linter --file "./ci/lint_imagefile" ./
-
-docker run --rm -v $(pwd):/bitcoin -it bitcoin-linter
+DOCKER_BUILDKIT=1 docker build -t bitcoin-linter --file "./ci/lint_imagefile" ./ && docker run --rm -v $(pwd):/bitcoin -it bitcoin-linter
 ```
 
-After building the container once, you can simply run the last command any time you
-want to lint.
+Building the container can be done every time, because it is fast when the
+result is cached and it prevents issues when the image changes.
 
 
 check-doc.py