This commit changes the launched pod to use the configured DOKKU_APP_SHELL - default /bin/bash - to launch processes. Without this, if a non-interactive process was executed and did not immediately exit, it was possible for that process to launch twice - once when the pod started, and once when entering the pod. Ideally we know whether the process is interactive or not, but this isn't always possible when running under a tty (non-tty == non-interactive). As such, this also removes the ability to launch one-off containers that do not have a configured shell in the container, but this is better than attempting to run commands twice.
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