Improve help messages for the command too.

The idea is that all the filtering capability should be replaced by
"grep" and shell pipes.

Part one (server still TODO).

Get rid of the useless / handler and move /webhook in its place.

Make them common to all commands, and clarify the docs about scoping
in general.

Support better integration with Gitlab CI, for automatic dependency
rebuilds via Gitlab Webhooks (minimal configuration required).

Update the documentation accordingly.

This ensures images are built in the right dependency order.