Support appending/editing startup commands to the launch script

[QuantumToasted]

Hi there, just want to start off by saying I appreciate the work that you and every other contributor has done to make this image a thing - as it's great for spinning up and distributing servers quickly versus working with something like LinuxGSM.

I would like to be able to append the +randommap command line argument instead of manually specifying +map ${SRCDS_STARTMAP} as defined here, which unfortunately is hardcoded. I'm unsure what the best way to do this would be, as I believe the commands are mutally exclusive, but I prefer my server being able to pick a random map out of the mapcyclefile as opposed to always starting with a specific map.

I'm unsure if it would add unnecessary complexity, but maybe the following two things could be simultaneously added:

• if SRCDS_STARTMAP is empty, don't append it to the startup command
• support maybe an additional SRCDS_EXTRAARGS envvar which is simply appended to the end, if possible?