[docker] Docker build management

[frgfm]

Hey there 👋

As discussed offline, we need to handle docker containers efficiently for this repo as it is critical for deployment.
Here is my suggestion:

• it is mandatory that the DockerHub repo https://hub.docker.com/repository/docker/pyronear/pyro-engine has the latest ARM images.
• CI docker push frequency: either we push at each merged PR, or at each release. I suggest doing those on releases, we can increase the release pace to use patch index (i.e. 0.1.1, 0.1.2, etc.)
• tag management : latest version should be pushed to the tag latest. Additionally, it should also be pushed to a descriptive tag: for instance, as of right now, the image is based on python:3.8.1-slim, so the tag should be python3.8.1-slim.
• device update: once pushed to dockerhub, a digest hash is created. This should be the reference for the software hash of the device.
• optionally, if we can manage to push to DockerHub the linux build as well (not overriding the ARM one), that would be nice 👍

Any suggestion is welcome!

[frgfm]

Closed by #106