Refactor Evaluator Images, Pipeline "Run" type image specification

[JersyJ]

• Reworked Evaluator Docker images (base/run/java/dotnet/clang-tidy/cargo/pythonrun)
• Kelvin: Updated pipeline configuration/validation to allow selecting a Docker image for the "tests" type (pipe).

[Copilot]

Pull request overview

This PR refactors several evaluator Docker images (base/run/java/dotnet/clang-tidy/cargo/pythonrun) and extends pipeline configuration/validation to allow selecting a Docker image for the tests pipeline action, with accompanying documentation updates.

Changes:

• Add an image option to the tests pipeline action (frontend validation + evaluator runtime support).
• Refactor evaluator Docker images (package installs, cleanup, pinned tooling versions, updated bleach).
• Update teacher documentation for pipeline/tests, including clearer code block formatting.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
frontend/src/PipelineValidation.js	Allows pipeline: - type: tests to accept an image field in config validation/help text.
evaluator/testsets.py	Minor warning formatting change (still within pipeline parsing error path).
evaluator/pipelines.py	Adds configurable image for TestsPipe and uses it to start the container.
evaluator/images/run/Dockerfile	Refactors apt install pattern and removes Java from the run image.
evaluator/images/pythonrun/Dockerfile	Re-bases pythonrun to kelvin/base and installs pytest/flake8 (but currently loses the run entrypoint behavior).
evaluator/images/java/entry.py	Removes runtime env-var injection logic (relies on Dockerfile env now).
evaluator/images/java/Dockerfile	Refactors installs and hardcodes Java/Maven env vars.
evaluator/images/dotnet/Dockerfile	Refactors installs; switches base reference to kelvin/base:latest.
evaluator/images/clang-tidy/Dockerfile	Switches from Alpine to kelvin/gcc and installs clang-tidy via apt.
evaluator/images/cargo/entry.py	Minor string formatting change in warning output.
evaluator/images/cargo/Dockerfile	Refactors apt install; bumps bleach version.
evaluator/images/base/Dockerfile	Consolidates packages, sets locale env, installs bleach 6.3.0 in base.
docs/docs/03-teachers-guide/02-task-configuration/05-pipeline.mdx	Documents the new tests.image option.
docs/docs/03-teachers-guide/02-task-configuration/03-tests.mdx	Documents tests.image and improves code block language annotations.

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[JersyJ]

Based on newest feedback from @Kobzol , I must postpone this, so I am putting this into Draft