Stop using latest and start using tagged version

Description

While latest is convenient, it's also usually not a good practice, as in general it brings unpredictability and security concerns.

In Docker images tags without shas are mutable, so in any case we'd have to use @sha to pin an image. But GitLab now provides mechanisms which also enhance security, like the container image tag protection feature, and soon immutability will be released also.

So we should start moving towards this direction, and pinning our images everywhere.

Proposal

To start being able to pin to a tagged version of issue-bot, let's introduce tags and releases.