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/release-tools/goreleaser.sh

@@ -29,6 +29,11 @@ if [ -z "${RELEASE_NOTES_FILE}" ]; then
     exit 1
 fi
 
+if [ -z "${REGISTRY}" ]; then
+    echo "REGISTRY must be set"
+    exit 1
+fi
+
 GIT_DIRTY=$(git status --porcelain 2> /dev/null)
 if [[ -z "${GIT_DIRTY}" ]]; then
     export GIT_TREE_STATE=clean