github: fix caching the docker images

[mkmkme]

This PR supercedes #170 and fixes #140

Pull Request check-list

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?

Description of change

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.26%. Comparing base (21f098c) to head (6c97033).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #176   +/-   ##
=======================================
  Coverage   76.25%   76.26%           
=======================================
  Files         130      130           
  Lines       33970    33970           
=======================================
+ Hits        25904    25906    +2     
+ Misses       8066     8064    -2     

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[mkmkme]

Expanded the commit message a bit:

This commit updates the way docker cache is created by including the
current date into the cache key. Now when "save-always" option in
actions/cache is deprecated [1] this seems to be the conventional
way [2] to create the cache.

1: actions/cache#1452
2. https://github.com/actions/cache?tab=readme-ov-file#creating-a-cache-key

Also removed the event_name == 'schedule' check since it doesn't make much sense with the current approach anymore. The cache will be created every day (like it should have been with the original approach). And the old cache will be removed after 7 days of not accessing it according to Github policy: https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/caching-dependencies-to-speed-up-workflows#usage-limits-and-eviction-policy

I am especially interested in seeing @smeso review on this PR because you created the original caching approach.

[mkmkme]

Hey @bogdanp05 , I've resolved our conversation. Your approach worked, thanks! Please approve if it looks good to you, it'll auto-merge.