ci: try to build deb package

[missytake]

Following the instructions at chatmail/relay#72 (comment)

[missytake]

Current error:

debconf: unable to initialize frontend: Dialog
debconf: (TERM is not set, so the dialog frontend is not usable.)
debconf: falling back to frontend: Readline
debconf: unable to initialize frontend: Readline
debconf: (This frontend requires a controlling tty.)
debconf: falling back to frontend: Teletype
Configuring pbuilder
--------------------

Default mirror not found

Mirror information detection failed and the user provided no mirror
information.

Please enter valid mirror information.

Please enter the default mirror you want to be used by pbuilder.

If you leave this field blank, there will be one attempt to autodetect this
information. If this attempt fails, you will be prompted again to insert some
valid mirror information.

Here is a valid mirror example: http://deb.debian.org/debian

Somehow --git-pbuilder-options="--mirror http://deb.debian.org/debian" didn't suffice to set a mirror.

[missytake]

https://staging.chatmail.at/build-area/dovecot-imapd_2.3.21+dfsg1-3_amd64.deb

woop woop

[link2xt]

This should ideally use actions/checkout so it checks out the PR commit and not whatever commit is on main now.
Apparently it should just work, the only problems users seems to have here are due to insufficient privileges:
https://stackoverflow.com/questions/68578638/how-to-checkout-repo-inside-the-docker-in-github-action

[missytake]

The problem is that the checkout action doesn't create the .git directory, but gbp expects it to be there...

Also, I'd only want to publish builds from main to https://download.delta.chat/dovecot/, as we don't have a staging environment where we could upload builds from other branches.

[link2xt]

The problem is that the checkout action doesn't create the .git directory

I just tested in #3, checkout action creates .git directory.

Now there is a problem that gbp is unhappy about changes on top of master (committing .github) that are not in the changelog. This will fail when merged on master, currently it only works because the workflow is not on the master branch yet.