ci(#1740): fix container workflow caching

[josecelano]

Summary

• fix checkout ordering and cache scoping in .github/workflows/container.yaml
• prebuild the tracker image in .github/workflows/testing.yaml and reuse it in E2E runners with --skip-build
• tighten .dockerignore and document the non-obvious cache-related Containerfile steps

Validation

• cargo check --bin e2e_tests_runner --bin qbittorrent_e2e_runner
• linter yaml
• linter all

This branch contains four separate commits:

• docs(#1740): add container workflow caching issue spec
• ci(container): fix checkout and cache scoping
• ci(e2e): reuse prebuilt tracker image
• chore(container): reduce context churn and document cache

[Copilot]

Pull request overview

This PR improves CI container build caching and speeds up Docker-based E2E jobs by ensuring Docker layer cache scopes don’t collide and by reusing a prebuilt tracker image instead of rebuilding it inside Rust runners.

Changes:

• Fix .github/workflows/container.yaml checkout ordering and scope BuildKit type=gha caches per target/job.
• Prebuild the tracker image in .github/workflows/testing.yaml and run E2E runners with --skip-build using the prebuilt tag.
• Reduce Docker context churn via .dockerignore updates and document cache-related/intentional Containerfile steps; add an issue spec doc describing the caching problem and solution.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/console/ci/qbittorrent_e2e/services_setup.rs	Adds skip_build plumbing to optionally skip docker compose build before bringing up the stack.
src/console/ci/qbittorrent_e2e/runner.rs	Adds --skip-build CLI flag and improves startup error context.
src/console/ci/e2e/runner.rs	Adds --tracker-image and --skip-build to reuse a prebuilt tracker image.
docs/issues/1740-fix-container-workflow-caching.md	Documents the caching issues/root cause and proposed workflow + context fixes.
Containerfile	Adds inline documentation for non-obvious build/cache-related steps.
.github/workflows/testing.yaml	Prebuilds a cached release image for Docker E2E and passes --tracker-image/--skip-build to runners.
.github/workflows/container.yaml	Fixes checkout ordering and scopes type=gha cache keys to avoid collisions across matrix/jobs.
.dockerignore	Excludes additional non-build files to reduce cache busting from unrelated changes.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.42%. Comparing base (20fca49) to head (57c1ed6).

Files with missing lines	Patch %	Lines
src/console/ci/compose.rs	0.00%	6 Missing ⚠️
src/console/ci/e2e/runner.rs	0.00%	5 Missing ⚠️
src/console/ci/qbittorrent_e2e/services_setup.rs	0.00%	5 Missing ⚠️
src/console/ci/qbittorrent_e2e/runner.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1741      +/-   ##
===========================================
- Coverage    79.45%   79.42%   -0.04%     
===========================================
  Files          368      368              
  Lines        27117    27129      +12     
  Branches     27117    27129      +12     
===========================================
+ Hits         21546    21547       +1     
- Misses        5290     5302      +12     
+ Partials       281      280       -1     

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