Support Immutable Tags

[amouat]

Some image tags should be changeable and some should be immutable. For instance:

zowie:latest
zowie:5
zowie:5.2

Should all be mutable - they may all point to zowie:5.2.8 at one point, then zowie:5.2.9 later.

However:

zowie:nightly-20-01-2020
zowie:git-35b29688db0
zowie:build-982315

Should all be immutable - they should only refer to single image over time. If they are overwritten with a new image, chances are it is a mistake or malicious. Kubernetes basically treats images as immutable, so this is important.

I believe Harbor and possibly quay already support this - have a look at how they've done it.