Image building instructions out of date

[petemoore]

https://github.com/mozilla/community-tc-config/edit/main/imagesets/README.md talks about the password store being on the mozilla internal git server. Now it lives elsewhere. Also the instructions should ensure that all keys are trusted that need to be, in order that pass commands do not fail. In fact, the imageset.sh script should probably take care of both gcloud auth login and checking gpg keys are all trusted before running, since this recently broke my run of imageset.sh.

Note instructions should include project requirements for gcloud configuration, such as .ssh config settings etc, so that pass git pull just works.

Another thing we could do, if the script needs to exit early, is to dump the remaining commands to standard err, so that an interrupted run can easily be completed post-failure. But that is just a possible optimisation, since when the script does exit early, it is a pain to work out what it hasn't done yet!