Preparing a new release based on a new Ubuntu codename

To rebase elementary OS on a new Ubuntu release, use the following process:

We will use a switch from Ubuntu jammy to Ubuntu lunar as an example.

Setting up Metapackage builds

1. Clone https://github.com/elementary/metapackages
   1. Create a new branch for the new Ubuntu release (e.g. lunar) from the previous Ubuntu release branch (e.g. jammy)
   2. In the new branch, in update.cfg replace instances of previous Ubuntu release with the new one.
   3. Commit and push new branch to repo
2. Clone https://github.com/elementary/seeds
   1. Create a new branch for the new Ubuntu release (e.g. lunar) from the previous Ubuntu release branch (e.g. jammy)
   2. In the new branch, in .github/workflows/update.yml replace instances of previous Ubuntu release with the new one.
   3. You may also need to change the container image that the workflow uses if the current one (usually rolling) doesn't point to a version that is the same or newer than the Ubuntu release we're trying to build against.
   4. Update the instances of the previous Ubuntu release with the new one in STRUCTURE
   5. Commit and push new branch to repo
3. Clone https://github.com/elementary/platform
   1. Create a new branch for the new Ubuntu release (e.g lunar) from the previous Ubuntu release branch (e.g jammy)
   2. In the new branch, in .github/workflows/update.yml replace instances of previous Ubuntu release with the new one.
   3. You may also need to change the container image that the workflow uses if the current one (usually rolling) doesn't point to a version that is the same or newer than the Ubuntu release we're trying to build against.
   4. Commit and push new branch to repo
4. Wait for the GitHub actions runs to complete on both repositories.
5. The new branch of https://github.com/elementary/metapackages should be committed to by the GitHub actions.
   1. Take note of any removed packages from these commits. This forms the list of packages that need to be built in Launchpad for the new codename. (e.g. https://github.com/elementary/metapackages/commit/bc133fb5d22a36574a00ec9d999e6ae7afc5e135)
6. Force an import of the metapackages repository on Launchpad: https://code.launchpad.net/~elementary-os/elementaryos/+git/metapackages
7. Create a new recipe from this repository pointing to the new codename branch, this should be named like seed-metapackages-lunar-daily
8. Enable the new codename for the overlay package and request a build: https://code.launchpad.net/~elementary-os/+recipe/os-patches-enabler
9. Enable the new codename for the icons package and request a build: https://code.launchpad.net/~elementary-os/+recipe/icons-daily
10. Wait for the builds to complete and publish! 🍵

Building the Docker container

1. Once the Launchpad builds above are completed, clone https://github.com/elementary/docker
2. Copy the previous unstable (horus-unstable) folder to a new codename (e.g. ubuntu-lunar-unstable)
3. Modify the Dockerfile in the new folder, replacing the old codename with new jammy -> lunar
4. Do a test build of the Docker image with:

docker build .

5. If there are no failures, add the new folder name to the lists in .github/workflows/ci.yml and .github/workflows/test.yml
6. Optionally symlink development-target to the new folder if you want every repository to be tested against this new docker image.
7. Push the changes to the repository

Building the important packages