Allow registry to be configured at build time

This adds a new `buildinfo` variable `ImageRegistry` that can set at
build time like the `Version` variable. This allows us to customise the
Velero binary to use different registries.

If the variable is set, this variable wille be used when creating the
URIs for both the main `velero` and `velero-restic-restore-helper`
images. If it is not set, default to using Dockerhub (`velero/velero`,
`velero/velero-restic-restore-helper`).

There are numerous ways in which the Velero binary can be built so all
of them have been updated to add the new link time flag to set the
variable:
* `make local` (used for local developer builds to build for the local
  OS and ARCH)
* `make build` (used by developers and also VMware internal builds to
  build a specific OS and ARCH)
* Goreleaser config (used when creating OSS release binaries)
* Dockerfile (used to build the Velero binary used within the image)

All of these workflows are currently triggered from our Makefile where
the variable `REGISTRY` is already available with the default value of
`velero` and used to build the image tag. Where the new `ImageRegistry`
build variable is needed, we pass through this Makefile variable to
those tasks so it can be used accordingly.

The GitHub action and the `./hack/docker-push.sh` script used to push
container images has not been modified. This will continue to use the
default registry specified in the Makefile and will not explicitly pass
it in.

Signed-off-by: Bridget McErlean <bmcerlean@vmware.com>

Makefile

@@ -110,7 +110,6 @@ GOPROXY ?= https://proxy.golang.org
 
 # If you want to build all binaries, see the 'all-build' rule.
 # If you want to build all containers, see the 'all-containers' rule.
-# If you want to build AND push all containers, see the 'all-push' rule.
 all:
 	@$(MAKE) build
 	@$(MAKE) build BIN=velero-restic-restore-helper
@@ -129,6 +128,7 @@ local: build-dirs
 	GOOS=$(GOOS) \
 	GOARCH=$(GOARCH) \
 	VERSION=$(VERSION) \
+	REGISTRY=$(REGISTRY) \
 	PKG=$(PKG) \
 	BIN=$(BIN) \
 	GIT_SHA=$(GIT_SHA) \
@@ -144,6 +144,7 @@ _output/bin/$(GOOS)/$(GOARCH)/$(BIN): build-dirs
 		GOOS=$(GOOS) \
 		GOARCH=$(GOARCH) \
 		VERSION=$(VERSION) \
+		REGISTRY=$(REGISTRY) \
 		PKG=$(PKG) \
 		BIN=$(BIN) \
 		GIT_SHA=$(GIT_SHA) \
@@ -186,6 +187,7 @@ endif
 	--build-arg=VERSION=$(VERSION) \
 	--build-arg=GIT_SHA=$(GIT_SHA) \
 	--build-arg=GIT_TREE_STATE=$(GIT_TREE_STATE) \
+	--build-arg=REGISTRY=$(REGISTRY) \
 	-f $(VELERO_DOCKERFILE) .
 
 container:
@@ -201,6 +203,7 @@ endif
 	--build-arg=VERSION=$(VERSION) \
 	--build-arg=GIT_SHA=$(GIT_SHA) \
 	--build-arg=GIT_TREE_STATE=$(GIT_TREE_STATE) \
+	--build-arg=REGISTRY=$(REGISTRY) \
 	--build-arg=RESTIC_VERSION=$(RESTIC_VERSION) \
 	-f $(VELERO_DOCKERFILE) .
 	@echo "container: $(IMAGE):$(VERSION)"
@@ -346,6 +349,7 @@ release:
 		GITHUB_TOKEN=$(GITHUB_TOKEN) \
 		RELEASE_NOTES_FILE=$(RELEASE_NOTES_FILE) \
 		PUBLISH=$(PUBLISH) \
+		REGISTRY=$(REGISTRY) \
 		./hack/release-tools/goreleaser.sh'"
 
 serve-docs: build-image-hugo