If the containers no longer exist and the images are unused, we should remove the images as well.
Note that images may sometimes stick around for a little while, depending on the retire schedule and the wait time specified for the container.
Closes#4104
This PR uses the new syntax for commands introduced in Docker 1.13, making it a bit easier to understand just what a particular command is trying to do.
This also pushes the long-form syntax for docker command flags, which are also easier to understand at a glance.
This change allows operators to specify a DOCKER_BIN environment variable. This will specify a binary to run when executing docker, which is useful in cases where the 'docker' command being run must be modified in a way that would otherwise be invasive to Dokku, but minimalistic if done within a wrapper.
Log output for ps:retire was previously a bit too verbose because we weren't handling the following edge-cases.
- if a container doesn't exist, remove it from the dead container list
- ensure a restarting container is properly killed
- only attempt to stop/kill if the state is not dead or exited
- do not attempt to do anything if the container doesn't exist
- correct check on running state
While I do not agree with _every_ style change, this will force Dokku to have consistent formatting across all shell scripts, which is arguably a Good Thing™.
The command used to reprocess everything is:
```shell
shfmt -l -bn -ci -i 2 -w .
```
These will be killed on a cron schedule, allowing us to ensure old containers don't run once memory pressure clears up - even across restarts.
Closes#3225
