Unknown Lagoon base image versions

[smlx]

Describe the bug
For about a week, the php:7.4-fpm-alpine image on docker hub has been incompatible with the lagoon php 7.4 Dockerfiles (see the fix in #1481).

The reason this hasn't been picked up by Lagoon CI, and builds have been successful in the past week, is that the Jenkins nodes have a cached php:7.4-fpm-alpine image from before the incompatible change was introduced.

This is a problem because it means that the base images that make up a Lagoon build are of unknown versions. They even differ between Jenkins nodes, so you could get different CI results and build artefacts depending on which node your job runs on.

To Reproduce
Steps to reproduce the behavior:

1. Go and look at all the successful builds of php 7.4 images on Lagoon CI in the last week.
2. Check out latest lagoon/master, and run docker image pull php:7.4-fpm-alpine && make build/php__7.4-fpm
3. Watch the build fail.

Expected behavior
The CI and/or build scripts should be building in a clean environment by removing all cached images and/or pulling all the base images at the start of a build. Bonus points for storing the hashes of the base images so that the build can be reproduced.

Screenshots
n/a

Additional context
I've got a partial fix in #1428, which I will continue working on.

In the meantime maybe it would suffice to docker system prune -af at the start of each CI job? Seems like it should work as long as jobs aren't running in parallel on the nodes.

[tobybellwood]

partial fixes in place - we now pin to upstream tags