Add alpine-latest helper image flavor and switch default alias to 3.15

What does this MR do?

Adds alpine-latest helper image flavor.

We also switch to using alpine3.15 by default as that's the latest as of writing this.

Why was this MR needed?

This is to allow people to always stay on the latest version in case that's a requirement.

What's the best way to test this MR?

• Check that the relevant jobs are being ran for alpine latest
• Since images are not being pushed in MRs you can easily build it locally:
• make out/helper-images/prebuilt-alpine-latest-x86_64.tar.xz
• In the build logs you should see Building helper image for: linux/amd64 based on alpine:latest
• docker import out/helper-images/prebuilt-alpine-latest-x86_64.tar.xz
• docker run --rm -it <image id> sh
• cat /etc/alpine-release should print 3.15.4 as of the time of writing this

What are the relevant issue numbers?

Closes #28896 (closed)