Fix docker labels

[nicobo]

Docker image labels must be key-pair values. E.g. :

• git.commit.sha=123456...
• source.url=https://github.com/...
• version=1.2.3
• maintainer=...@...

Make also sure they are in all image variants.

Complete existing labels with all required for filtering (can filter on both key and key + value). E.g. filter by :

• signal-enabled
• base image (redundant?)
• arch (redundant?)

Current situation :

"Labels": {
                "0.1.dev1": "",
                "96218c008e1b01009f49ff4578570b80dfebdfc6": ""
            }