Adds tag for docker builds

[spencerrlongg]

Description

Adds new tags for v{{major}}-latest and v{{major}}-latest-alpine

We may want to do {{version}} instead to get minor & patch numbers maybe?

Fixes #12853

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Untested at the moment, not sure if it needs to be, I don't have docker set up on this machine but can do that if I need to.

[what-the-diff]

PR Summary

• Add SemVer tag for docker-alpine.yml
  Introduce a versioning system to manage the docker-alpine build.
• Add SemVer tag for docker.yml
  Implement a versioning system to better manage the main docker build.

[snipe]

@DanielWeigl care to weigh in here?

[snipe]

Ping @DanielWeigl - can you put eyes on this when you get a moment?

[DanielWeigl]

Hi, sorry for the late response, yes the changes look correct, but I don't have any experience with the GitHub build chain. So, im not sure if it will work as intented.

[snipe]

@spencerrlongg can you let me know if this is ready to be taken out of WIP?

[spencerrlongg]

@uberbrady would you mind taking a look at this and seeing if it makes sense to you?

[uberbrady]

This seems pretty dang reasonable to me @spencerrlongg - as I understand it, it means that we'll now have 'v6-latest-alpine' and 'v6-latest' for those two particular builds - and when v7 comes out, we'll also have 'v7-latest-alpine' and 'v7-latest' - if that's what you're going for here, then that sounds good to me!

[spencerrlongg]

Alright, that's what they were asking for in the initial linked issue, so I'll mark ready for review, easy enough to revert if we have unintended consequences.