Docker workflow: keep .git in build context

By default, BuildKit checks out the git repository but does not include the
.git directory in the build context. This then prevents go build from embedding
the commit hash into the binary which we want to include in the --version
output. This behavior can be changed by setting a build argument and fixes our
--version output.

.github/workflows/docker.yml

@@ -39,3 +39,7 @@ jobs:
         platforms: linux/amd64,linux/arm64
         push: ${{ github.event_name != 'pull_request' }}
         tags: ${{ steps.meta.outputs.tags }}
+        # Keep the .git to allow including the commit in the --version output, see also:
+        # https://docs.docker.com/build/building/context/#keep-git-directory
+        build-args: |
+          BUILDKIT_CONTEXT_KEEP_GIT_DIR=1