Extract generic shell command from docker command

What does this MR do and why?

Extracts the code for executing shell commands from Docker::Shellout to Support::ShellCommand so that it can also be used in non-Docker contexts.

This is needed for the changes in !1024 (merged) (and these changes were originally included in that MR but I split them off to their own MR here).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.