Add caching of image files to build process

[jaydrogers]

Background

• @szepeviktor was gracious and contributed caching
• I temporarily commented this out in the actions file to solidify the build before

Caching will be helpful because:

1. Saves time in CI
2. We are good internet citizens by not draining @oerdnj's bandwidth resources

Scenarios to consider

✅ When we want a build

• There is an update to the base_image (ubuntu:20.04)
• There is an update in @oerdnj's repository

🛑 DO NOT build when

• There are no updates for @oerdnj's repo

(notice we still need to build even if there are not updates for the base_image)

Possible solutions

1. Check hash of @oerdnj's repo (http://ppa.launchpad.net/ondrej/php/ubuntu/dists/focal/Release)
2. Something like docker run ... /bin/bash -c "dpkg -l|shasum"

Related comment: 26723e7#r48044607

Next steps

• Finish the structuring (Start building out PHP images #9) first, then loop back to this

[szepeviktor]

Cache (the hash of) this file: http://ppa.launchpad.net/ondrej/php/ubuntu/dists/focal/Release

Done 🍏

[szepeviktor]

Please send the usual €1 000 000

[szepeviktor]

Actually building the image and checking it for changes would be the ultimate solution.
But timestamps make it impossible.

But you may experiment with docker run ... /bin/bash -c "dpkg -l|shasum"

[jaydrogers]

Adding a note, I wonder if we use "Build X" (BuildKit) as provided in this demo? https://github.com/docker/build-push-action

[szepeviktor]

I wonder if we

To answer that you need someone who lives in the Docker ecosystem.

[jaydrogers]

Updated the notes above. Temporarily commented out the caching for now and will loop back to this later.

Focusing on the structure for now.