Add support for Windows dial-in commands

[AlvaroRausell]

This PR enables the user to add a custom Windows command to use for container dial-in, as opposed to the current hard-coded behaviour. On the agent template page, there is now a text area called Windows Command (the former Command text area has been renamed to Unix Command. The motivation behind this is my team's need to allow the containers to connect via cygwin as opposed to powershell. I also made it so one can write ${unixCommand} on the Windows Command field to interpolate the default bash behaviour without needing complicated parsing.

Closes issue #53.

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[AlvaroRausell]

@Roemer Would it be possible for you to have a look into it whenever you can please?