[DOC]: Pin docker image with hash

[MacOS]

This PR pins the docker image used in docs with a hash to increase security.

The OpenSSF Score should increase after this PR has been merged.

The PR is linked to #594 and #814 and #815. I did not open an issue for this PR because the change is a little to small, and #815 is similar in spirit as it pins actions with the hash.

[Copilot]

Pull request overview

This PR increases supply-chain security for the documentation build container by pinning the node:20-alpine base image to an immutable SHA256 digest in docs/.docker/Dockerfile, aligning with the broader OpenSSF hardening work referenced in the PR description.

Changes:

• Pin node:20-alpine base image to a specific sha256 digest in the docs Antora Dockerfile.

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[michalharakal]

Tnank you @MacOS

Note: the failing preview-documentation check is not related to this change — it's a known limitation where fork PRs get a read-only GITHUB_TOKEN, so the "post preview comment" step can't run. Docs build fine and the artifact uploads correctly. Safe to ignore here; a fix to skip that step on fork PRs is coming separately.