Allow overwriting default Dockerfile location

Allow the usage of a non-default Dockerfile name and/or location.

In some cases (e.g. a monorepo) the Dockerfile isn't always in the repo's base directory or could even have a different name. Being able to overwrite the default Dockerfile name and/or location the build variable CI_DOCKERFILE was introduced.

If unset the current behaviour is retained.

I'm uncertain about the variable naming, in case please propose a different name.