Tagging policy

[pauleve]

When adding a git tag, hub.docker builds the corresponding image with a persistent tag.
This is useful to make reference to the exact same docker image used for a model analysis.
Tags should be understood as versions of the docker image.

First proposal: the tags are of the form YYYY-MM-DD, and are created after each bunch of changes in the Docker which affect the shipped tools [of course done at most once a day].