Allow for building Dockerfiles from develop branch of Gaffer

[GCHQDeveloper314]

It would be good to support building images directly from the develop branch of Gaffer. Originally posted by @GCHQDeveloper314 in https://github.com/gchq/gaffer-docker/issues/325#issuecomment-1704992927

This isn't possible because the GAFFER_VERSION ARG needs to be both a valid Gaffer version (for copying JAR files) and also a branch name.

https://github.com/gchq/gaffer-docker/blob/f13bb50e56e6faa236688bea4bf6401f2966eeb2/docker/gaffer/Dockerfile#L49

https://github.com/gchq/gaffer-docker/blob/f13bb50e56e6faa236688bea4bf6401f2966eeb2/docker/gaffer/Dockerfile#L53