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 #325 (comment)

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.

gaffer-docker/docker/gaffer/Dockerfile

Line 49 in f13bb50

git checkout ${GAFFER_VERSION} || git checkout gaffer2-${GAFFER_VERSION} && \

gaffer-docker/docker/gaffer/Dockerfile

Line 53 in f13bb50

if [ ! -f "/jars/accumulo-store-${GAFFER_VERSION}-iterators.jar" ]; then \

[tb06904]

There is an alternative to this where instead we could use Maven to download the Gaffer JARs and copy them into the image (how the current gaffer-gremlin image does it). This technically allows develop building as you can clone and locally build Gaffer as a snapshot which will be available in the local Maven repo so could be specified as a version for copying into the image.