ci(bluebuild): override CLI container entrypoint to bluebuild binary

Container's default entrypoint is dumb-init, which interpreted 'build'
as a command to exec rather than as a bluebuild subcommand. Pin
--entrypoint /usr/local/bin/bluebuild and pass 'build ...' as args.

.github/workflows/build-bluebuild.yml

@@ -108,6 +108,7 @@ jobs:
           # Mount the repo + podman socket; build with buildah driver.
           podman run --rm \
             --privileged \
+            --entrypoint /usr/local/bin/bluebuild \
             -v "$PWD:/work" \
             -w /work \
             -e BB_BUILD_DRIVER=buildah \