[HWORKS-2991][APPEND][HWORKS-3040] Correct conda-era docs claims, and document installing a compiler

[robzor92]

Two commits.

[HWORKS-2991][APPEND] Two docs claims left over from the conda era

The uv migration is already documented — the retired conda / .egg / environment.yml installs in python_install.md, and export-is-not-an-import in python_env_export.md. Two claims elsewhere were missed:

• concepts/dev/inside.md still listed conda channels as a source you can install dependencies from. The backend refuses that install, and refuses a conda-channel search too.
• environment_history.md said the per-version YAML file is kept "so that if you want to restore an older environment you can use it". Importing an environment.yml is no longer supported and the history API is read-only (getAll / getEnvironmentDelta, no restore endpoint), so the file is a record and nothing more.

HWORKS-3040 Installing a compiler

Documentation for docker-images#926, which removes build-essential and the -dev packages that pull in the kernel headers from the base images. Asked for on Slack as the condition for merging it: document the compiler issue, with a custom commands example for installing one.

One new section: which environments have a compiler and why the others do not, what the failure looks like, and a script that installs one, builds the library and purges it again so the environment does not end up carrying the kernel headers either. Plus when not to purge — more installs to follow, a -dev package that owns a shared library the wheel loads, or the PyTorch and Ray environments, which use their compiler at runtime.

Also BUILD_PATH → BUILD_DIR in the artifacts bullet: the generated Dockerfile sets BUILD_DIR, and nothing sets BUILD_PATH.

Verified end to end on a cluster

The python-feature-pipeline base image in a cluster's registry was replaced with one built from docker-images#926 (gcc, cc, build-essential and linux-libc-dev all absent, confirmed from an imagePullPolicy: Always pod), and a project environment was cloned from it.

1. Installing thriftpy2==0.5.2 from the library API fails. The environment build log ends with error: [Errno 2] No such file or directory: 'cc', under uv's × Failed to build framing — the environment build installs with uv, so that is the error quoted here rather than pip's.
2. The script in the docs, uploaded to HopsFS and run as a custom command, succeeds. The environment goes to version .1 with thriftpy2 0.5.2, cython 3.3.0 and ply 3.11 installed.
3. The resulting environment image is clean. No gcc, no cc, no build-essential, linux-libc-dev uninstalled, while thriftpy2 imports and its compiled cpython-312-x86_64-linux-gnu.so extensions are present and load. hopsworks, pandas and psycopg2 still import, and /etc/sudoers.d holds only README, so the sudo grant really is build-scoped as documented.

The script exactly as written here was also run verbatim as yarnapp, with the same /etc/sudoers.d/yarnapp grant the build creates, and exits 0 with the same end state.

markdownlint-cli2 is clean. Backports to branch-5.0 / branch-4.8 are owed whenever #926 is backported, and not before.

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