Put Alpine and Ruby versions back in single.Dockerfile
@phillipwells ran into this issue while doing the 15.7 release:
I'm doing the docs release for 15.7. It seems like Docker is having a hard time grabbing the Ruby and Alpine versions from .gitlab-ci.yml. The pipeline fails pretty early on:
Step 5/25 : FROM ruby:${RUBY_VERSION}-alpine${ALPINE_VERSION} AS builder invalid reference format
Looks like this goes back to a change from about a week ago: !3370 (merged). Thanks in advance for the help!
This MR puts the values for RUBY_VERSION and ALPINE_VERSION back in single.Dockerfile for now. We can look into why variables didn't pass as expected from .gitlab-ci.yml when we're back after the holidays.