Update to Docker components based ideas from @kellrott for distributing images across clusters.

The --docker_image_cache option to docker_build was already there - but I have tweaked the namespace for the files and updated the Galaxy job script dependency resolution stuff to actually load and use the cached images if they are avaialble thanks to a script from @kellrott.

Use the following command to cache docker images for Galaxy during development.

planemo docker_build --docker_image_cache ../galaxy-central/database/container_images [TOOL_PATH_OR_DIRECTORY]

Matching changeset in Galaxy is here - https://bitbucket.org/galaxy/galaxy-central/commits/a3cff55b129fcbf9b8e359ff0f71cb844a7e9277.