Fix setuptools-scm version in container image

[pederhan]

Fix container image versioning for tagged releases

Problem:
When building container images from tagged commits, setuptools-scm incorrectly appends commit hash and date to the version number. This happens because our build process creates requirements.txt, as well as possibly modifying uv.lock, causing setuptools-scm to detect the repository as "dirty" even though we're building from a clean tagged commit.

Solution:

1. Add requirements.txt to .gitignore to prevent it from being tracked
2. Set UV_FROZEN=1 environment variable to prevent any modifications to the lock file during build
3. Update lock file to match dependencies in pyproject.toml (was missing coverage[toml] from Further coverage fixes, now with mocks. #568).

This ensures that container images built from tags (e.g., v1.2.3) will have clean version numbers that match the git tag instead of versions with appended hashes (e.g., 1.2.3.dev5+g123abc).

[terjekv]

LGTM (for loose enough versions of "good" that can be applied to python package management)

[coveralls]

coverage: 99.022%. remained the same
when pulling a721e30 on pederhan:fix-scm-version
into 43c9b5c on unioslo:master.