build: pin pixi version in images/Dockerfile

[InessaPawson]

What does this implement/fix?

Pins the pixi version installed in images/Dockerfile.

Previously the installer ran unpinned, so every image build picked up whatever the latest pixi release was that day. A pixi minor bump can change lock handling or resolution behaviour with no corresponding change in this repo, making builds non-reproducible.

This PR:

• Adds a PIXI_VERSION build arg (currently v0.81.0) in the builder stage and passes it to the installer, so the version is explicit and bumped deliberately.
• Adds a post-install check (pixi --version | grep -qF "${PIXI_VERSION#v}") that fails the build if the installed binary doesn't match the pin.

No other lines change. Later stages still copy /usr/local/bin/pixi from the builder as before.

Not included: automated bumps. Dependabot's docker ecosystem only tracks FROM images, not build args, so keeping this current would need a Renovate custom regex manager. Happy to open that as a follow-up if wanted.

Closes #243.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe)

Testing

• Did you test the pull request locally?
• Did you add new tests?

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[pmeier]

Thanks Inessa!