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>

hack/docker-push.sh

@@ -91,6 +91,9 @@ echo "BUILDX_PLATFORMS: $BUILDX_PLATFORMS"
 
 echo "Building and pushing container images."
 
+# The use of "registry" as the buildx output type below instructs
+# Docker to push the image
+
 VERSION="$VERSION" \
 TAG_LATEST="$TAG_LATEST" \
 BUILDX_PLATFORMS="$BUILDX_PLATFORMS" \