Add No docker prune env var

Pishelrequested to merge
pishel65/gitlab-runner:fix/issue-39415-no-docker-prune-env-var into main

What does this MR do and why?

Closes #39415 (closed)

During apt/rpm package install or upgrade, the post-install scripts unconditionally call clear-docker-cache prune. This is surprising behaviour: users who do not want docker pruned during an upgrade have no way to opt out, and the prune can stall upgrades for a long time.

Root cause

packaging/scripts/postinst.deb and packaging/scripts/postinst.rpm call /usr/share/gitlab-runner/clear-docker-cache prune with no guard, so it always runs.

Fix

Follow the existing USE_INIT_USER pattern in post-install: check for an env var (NO_DOCKER_PRUNE) and skip the prune when it is set. The default behaviour (prune runs) is preserved so existing users who rely on it are not broken.

To skip the prune for a single upgrade:

NO_DOCKER_PRUNE=1 apt-get upgrade gitlab-runner

To skip it permanently (until the env var is unset in the calling environment), export it in the shell or init system that invokes the package manager.

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