A change in cloud-init and/or the Ubuntu image provided by Digitalocean causes builds to fail due to cloud-init status exiting 2. The error is ignorable.
See digitalocean/marketplace-partners#186 for more details.
Several of the packages being installed were actually transitional packages, that is, the package itself is either a complete no-op (since the package is already installed), or else the package only exists to depend on another package of a new name (to support people who are running older operating systems).
For the former, we can drop the dependency entirely, and for the latter we should use the new package name directly.
Through some random machinations, software-properties-common now has a dependency on systemd. This breaks the built docker image as plugins will think systemd is available, so `dokku plugin:install --core` will reference that over sv.
Since we don't manage ppas in Dokku anymore - the only use was for an nginx ppa so we could use a newer Nginx - this can be safely removed.
Also remove some other stuff related to systemd that aren't required anymore.
This change converts existing CHECKS files into a healthchecks key that is understood by 'docker-container-healthchecker'. This tool supports a number of different types of container healthchecks - command, http, uptime - and can perform healthchecks against non-web processes.
The use of the old CHECKS file is now deprecated, and will be removed in the next minor version. Users can use the 'docker-container-healthchecker' to convert existing CHECKS files to the new format automatically.
Closes#2760
This makes standard use of shellcheck work without needing to provide extra configuration anywhere.
Also remove use of inline 'shellcheck disable' calls that are already defined in the .shellcheckrc and don't need to be set inline.
