make buildx: fix Makefile version being ignored

The `BUILDX_COMMIT` variable was set as a Make variable,
which isn't exported, and thus not available in scripts,
unless referenced through `$(VAR)` (non-curly-brackets).
As a result `--build-arg BUILDX_COMMIT` did not set the
`BUILDX_COMMIT` build-arg, and the default from the Dockerfile
(`master`) was used instead.

This patch exports the default version that's set in the
Makefile, so that it can be used as a regular environment
variable. The script was also slighly modified to no longer
use the `Make` variable.

In addition, the `buildx` target now calls `buildx version`,
which is useful to confirm if the binary was successfully built
(and with the correct version).

Before:

    rm -f bundles/buildx && make buildx && ./bundles/buildx version
    # => => naming to docker.io/library/moby-buildx:v0.3.0
    github.com/docker/buildx v0.3.1 6db68d0

    # using a make variable:
    rm -f bundles/buildx && make BUILDX_COMMIT=v0.2.1 buildx && ./bundles/buildx version
    # => => naming to docker.io/library/moby-buildx:v0.2.1
    github.com/docker/buildx v0.3.1 6db68d0

    # using an environment variable:
    rm -f bundles/buildx && BUILDX_COMMIT=v0.2.2 make buildx && ./bundles/buildx version
    # => => naming to docker.io/library/moby-buildx:v0.2.2
    github.com/docker/buildx v0.3.1 6db68d0

After:

    # default
    rm -f bundles/buildx && make buildx
    # => => naming to docker.io/library/moby-buildx:v0.3.0
    github.com/docker/buildx v0.3.0 c967f1d

    # using a make variable:
    rm -f bundles/buildx && make BUILDX_COMMIT=v0.2.1 buildx
    # => => naming to docker.io/library/moby-buildx:v0.2.1
    github.com/docker/buildx v0.2.1 0eb2df5

    # using an environment variable:
    rm -f bundles/buildx && BUILDX_COMMIT=v0.2.2 make buildx
    # => => naming to docker.io/library/moby-buildx:v0.2.2
    github.com/docker/buildx v0.2.2 ab5fe3d

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

1 files changed, 14 insertions, 5 deletions

diff --git a/Makefile b/Makefile
index a075f64aaa..a7633a1694 100644
--- a/Makefile
+++ b/Makefile
@@ -252,16 +252,25 @@ else
 buildx:
 endif
 
+BUILDX_COMMIT ?= v0.3.0
+export BUILDX_COMMIT
+
 bundles/buildx: BUILDX_DOCKERFILE ?= Dockerfile.buildx
-bundles/buildx: BUILDX_COMMIT ?= v0.3.0
 bundles/buildx: bundles ## build buildx CLI tool
 	# This intetionally is not using the `--output` flag from the docker CLI which is a buildkit option
 	# The idea here being that if buildx is being used, it's because buildkit is not supported natively
-	docker build -f $(BUILDX_DOCKERFILE) -t "moby-buildx:$(BUILDX_COMMIT)" \
+	docker build -f $(BUILDX_DOCKERFILE) -t "moby-buildx:$${BUILDX_COMMIT:-latest}" \
 		--build-arg BUILDX_COMMIT \
 		--build-arg BUILDX_REPO \
 		--build-arg GOOS=$$(if [ -n "$(GOOS)" ]; then echo $(GOOS); else go env GOHOSTOS || uname | awk '{print tolower($$0)}' || true; fi) \
 		--build-arg GOARCH=$$(if [ -n "$(GOARCH)" ]; then echo $(GOARCH); else go env GOHOSTARCH || true; fi) \
-		. && \
-		id=$$(docker create moby-buildx:$(BUILDX_COMMIT)); \
-		if [ -n "$${id}" ]; then docker cp $${id}:/usr/bin/buildx $@ && touch $@; docker rm -f $${id}; fi
+		.
+
+	id=$$(docker create moby-buildx:$${BUILDX_COMMIT:-latest}); \
+	if [ -n "$${id}" ]; then \
+		docker cp $${id}:/usr/bin/buildx $@ \
+		&& touch $@; \
+		docker rm -f $${id}; \
+	fi
+
+	$@ version