refactor: remove rules_docker dependency

[alexeagle]

It was only here for a container test, but we have replaced this functionality with py_image_layer which is based on rules_oci.

• Merge the code of containers/ example with the py_image_layer example
• Test it as an actual OCI image, rather than just a golden test of the archive listing
• remove the "manual" tag from the container_structure_test

Pre-factoring for switching to bzlmod; we don't want to try loading rules_docker from MODULE.bazel

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Changes are visible to end-users: no

Test plan

• Covered by existing test cases

[github-actions]

LCOV of commit 9677f3f during CI #1625

Summary coverage rate:
  lines......: 100.0% (2 of 2 lines)
  functions..: 100.0% (1 of 1 function)
  branches...: no data found

Files changed coverage rate: n/a

[aspect-workflows]

Test

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29 tests (100.0%) were fully cached saving 55s.