For internal builds of Velero, we need to be able to specify an
alternative Dockerfile which uses an alternative image registry to pull
the base images from. This change adapts our Makefile such that both the
main Dockerfile and build image Dockerfile can be overridden.

We have some special handling for the build image to only build when the
Dockerfile has changed. In this case, we check whether a custom
Dockerfile has been provided, and always rebuild in that case. For
custom build image Dockerfiles, use a fixed tag rather than the one
based on commit SHA of the original file.
Signed-off-by: Bridget McErlean <bmcerlean@vmware.com>