Free up Docker image tag namespace.

Free up Docker image tag namespace.

Allow tags to distinguish Docker images for different release
requirements. Free up the image tag namespace for granularly describing
image revisions (and create smaller image repositories) by using the
matrix slug as the image name. Tag images to indicate the supported
GROMACS branch.

Refs #3395

Change-Id: Idff95d031cd56a28545562862e78cd99902db620