fix: installer review follow-ups

- install.sh saves the get.docker.com and uv installers to a file and runs
  them only after the download finished, so a cut-off transfer runs nothing.
- Neither installer prints DOCSGPT_PACKAGE, which may be a URL with
  credentials.
- The CI step assigns the wheel path before exporting it, so a missing wheel
  fails instead of installing from PyPI.
- Docker-Deploying shows one code block per platform; Quickstart names the
  /opt/docsgpt home used for root on Linux.

.github/workflows/docker-image-verify.yml

@@ -108,7 +108,9 @@ jobs:
           # Same Compose project name as the step above; stop that stack first.
           docker compose -f deployment/docker-compose-standalone.yaml down -v
           pipx run build --wheel --outdir "$RUNNER_TEMP/dist"
-          export DOCSGPT_PACKAGE="$(ls "$RUNNER_TEMP"/dist/docsgpt-*.whl)"
+          # Assigned before export, so a missing wheel fails here instead of installing from PyPI.
+          DOCSGPT_PACKAGE="$(ls "$RUNNER_TEMP"/dist/docsgpt-*.whl)"
+          export DOCSGPT_PACKAGE
           # No uv is set up beforehand, so the installer's pinned uv download runs too.
           # Without a terminal the installer passes --yes to docsgpt up.
           bash deployment/install.sh --image-tag verify </dev/null