Defer assignment of Docker gcr.io secret

With the ':=' assignment in the Makefile, the SECRET was
evaluated at the start, regardless of whether it was needed
or not to complete the recipe.

This causes an issue if you don't have the necessary
docker-credential-gcr tool which is expected to be there
for Jenkins.

This change defers the evaluation by using '=' instead and
avoids confusing error messaages when building locallly.

Signed-off-by: John Schnake <[email protected]>

Makefile

@@ -28,8 +28,10 @@ LINUX_ARCH := amd64 arm64
 DOCKERFILE :=
 PLATFORMS := $(subst $(SPACE),$(COMMA),$(foreach arch,$(LINUX_ARCH),linux/$(arch)))
 
-# SECRET is used when pushing the manifest.
-SECRET := $(shell echo "https://gcr.io" | docker-credential-gcr get | jq '.Secret')
+# SECRET is used when pushing the manifest. docker-credential-gcr may not be
+# available locally (except in Jenkins) so delay the evaluation unless the secret
+# is needed.
+SECRET = $(shell echo "https://gcr.io" | docker-credential-gcr get | jq '.Secret')
 
 # Not used for pushing images, just for local building on other GOOS. Defaults to
 # grabbing from the local go env but can be set manually to avoid that requirement.