Add step to delete all migrations images from docker cache

[mikaylathompson]

Description

We believe that the docker cache is causing the e2e tests to fail, possibly by taking up too much disk space and forcing the target cluster to lock indices.

One solution (assuming a correct diagnosis) is to delete docker caching altogether. This, of course, also means we lose the benefits of caching the external images (namely, not being rate limited). This PR is an attempt to split the difference--keep the cache for external images, but don't cache all the migrations/ images, which we're rebuilding anyways.

Issues Resolved

Hopefully https://opensearch.atlassian.net/browse/MIGRATIONS-1797

Testing

Several tests in the commits to this PR.

• The first commit is the actual content.
• The second commit makes a change to a Dockerfile and therefore creates a fresh cache (see run here: https://github.com/opensearch-project/opensearch-migrations/actions/runs/9588463478/job/26440545131). In this one, you can see the delete images step running: https://github.com/opensearch-project/opensearch-migrations/actions/runs/9588463478/job/26440545131#step:11:1; and creating a cache entry with a smaller number of images and size: https://github.com/opensearch-project/opensearch-migrations/actions/runs/9588463478/job/26440545131#step:18:28
• The third commit is an empty commit which means it will have a cache hit on the cache created by the second commit. (see run here: https://github.com/mikaylathompson/opensearch-migrations/actions/runs/9588586214/job/26440929280). It pulls a much smaller set of images than previously: https://github.com/mikaylathompson/opensearch-migrations/actions/runs/9588586214/job/26440929280#step:7:35, and the tests pass: https://github.com/mikaylathompson/opensearch-migrations/actions/runs/9588586214/job/26440929280#step:10:46 🎉

Before merging, I intend to roll back the final two commits.

Check List

• New functionality includes testing
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.90%. Comparing base (1488d90) to head (0501b56).
Report is 6 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #748      +/-   ##
============================================
+ Coverage     64.75%   64.90%   +0.14%     
- Complexity     1585     1586       +1     
============================================
  Files           238      239       +1     
  Lines          9880     9919      +39     
  Branches        771      771              
============================================
+ Hits           6398     6438      +40     
  Misses         3072     3072              
+ Partials        410      409       -1     

Flag	Coverage Δ
gradle-test	64.03% <ø> (+0.04%)	⬆️
python-test	74.45% <ø> (+0.88%)	⬆️

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[mikaylathompson]

This PR itself seems like additional evidence towards docker cache being the issue. Testing with a cache hit failed, cache miss succeeded.

The cleanup step worked as intended: https://github.com/opensearch-project/opensearch-migrations/actions/runs/9588463478/job/26440545131?pr=748#step:18:28