Update packaging scripts to build in Docker

[palpant-dbx]

This change reworks the packaging scripts to build inside Docker, since we can no longer build for both Ubuntu and Fedora on an Ubuntu machine.

• Dockerfiles for bare Ubuntu, Debian, Debian (i386) and Fedora images are added with necessary build dependencies
• Two new build scripts are created to replace build_packages.py:
  • build_packages.sh: builds all DEB and RPM packages in Docker containers
  • build_repos.sh: builds the apt and yum repos including these packages
• Since we are building in clean Docker containers under the appropriate OS, we no longer need an additional chroot for each build, so we simplify the scripts to call debuild and rpmbuild directly rather than via pbuilder and mock
• All build results are output to build/ and this directory is added to .gitignore
• HOWTO_PACKAGE.md is updated with the new procedures to follow
• Some files related to RPM signing are removed since this will be handled internally
• Fedora 39 is added to distro-info.sh since it has begun development

In our production builds (internally), we will first run build_packages.sh, then sign the packages, then run build_repos.sh and tar up all the build artifacts.

Tested by running:

$ ./build_packages.sh
$ ./build_repos.sh

[jewelpit]

LGTM, other than the git clean part :)

[jewelpit]

Could we either remove this or make it an interactive clean?

[palpant-dbx]

I'm also in favor of removing it. I suspect it was included in the original build_packages.py since this had to be run locally, but since we are moving to CI, where the checkout will always be clean, there is no value for release builds, and it's fairly anti-social if you're just testing locally.

[jewelpit]

Is this necessary?

[palpant-dbx]

It's not strictly necessary as this is run inside Docker. I added it so that if you run the script locally on your machine (outside Docker), it leaves you in the same directory that you started (the top-level repo root). I'll add a comment, but am happy to remove it if it causes some other problem

[jewelpit]

I'm so glad to see the .rpmmacros stuff gone!