fix(ci): harden the Docker release path

[shauneccles]

Problem

The Docker publish workflow interpolates the manual version input directly into Bash source before parsing. It also restores and writes a shared GitHub Actions build cache in the release job that publishes to GHCR.

That gives a workflow input a shell parsing boundary and lets cache state produced outside the release run influence the published image build.

Change

• pass the version input through a step environment variable
• write GITHUB_ENV with quoted printf arguments
• remove type=gha cache restore and save from the image-publishing step

The image tags, platforms and registry permissions are unchanged. The trade-off is a slower release build with no cross-run cache trust.

Verification

• workflow YAML parsed successfully
• the zizmor template-injection findings are removed
• no GHA cache remains in the Docker publish path
• git diff --check passed

Action pinning and checkout credential persistence are deliberately left for the follow-up repository-wide pinning PR.