Sanitize the default docker image tag name: git branches can contain invalid characters
Sanitize the default docker image tag name: git branches can contain invalid characters

Previously it also lowered the branch name, which should never have
been done: it could create collisions, which could delete official
images.

Now sanitize_name_unique guarantee uniqueness even in case of sanitation.

Deepomatic/dmake,Deepomatic/dmake,Deepomatic/dmake,Deepomatic/dmake