Add the missing allow_failure, without this the badge will not get
updated if a job fails.

Related: https://gitlab.alpinelinux.org/alpine/apk-tools/-/issues/10843

Alpine released 3.16 recently so postmarketOS will have soon
v22.06 ready as the latest stable version. Adjust the CI to
test upgrades from v21.12 to v22.06 and v22.06 to edge.

Allow failures in the upgrade stage to make sure the pipeline
badge is updated.

Verify with shellcheck if all shell scripts are POSIX compliant

Initial CI which is run daily to verify if we can upgrade
without any issues from stable to edge.