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
This adds a table of environment variables that Dokku will use to change internal behavior.
Not all are fully documented/listed, but this should be enough to get people hacking.
[ci skip]
