This functionality was broken due to a refactor of argument handling in a previous release.
In addition to the fix, all the functionality was moved to scheduler-enter, which allows scheduler plugins to implement ways of entering containers in the relevant scheduler.
Closes#3972
This standardization makes it somewhat easier to read stacktraces as the command names are all uniform, so it will be slightly easier to scan trace output.
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.
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 .
```
This allows developers to change the shell from /bin/bash to any other when executing
`dokku enter` or pre/post deployment tasks. Ideally we could detect the shells available,
but that is slow and potentially error prone.
Closes#2572
