Fix netstandard runtime error by installing mono-devel

[HebaruSan]

Problem

# mono /usr/local/bin/KSPMMCfgValidator.exe

Unhandled Exception:
System.TypeInitializationException: The type initializer for 'KSPMMCfgValidator.KSPMMCfgValidator' threw an exception. ---> System.BadImageFormatException: Could not resolve field token 0x0400000a, due to: Could not load type of field 'KSPMMCfgValidator.KSPMMCfgValidator+<>c:<>9__1_1' (1) due to: Could not load file or assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies. assembly:/usr/local/bin/KSPMMCfgValidator.exe type:<>c member:(null)
   --- End of inner exception stack trace ---
[ERROR] FATAL UNHANDLED EXCEPTION: System.TypeInitializationException: The type initializer for 'KSPMMCfgValidator.KSPMMCfgValidator' threw an exception. ---> System.BadImageFormatException: Could not resolve field token 0x0400000a, due to: Could not load type of field 'KSPMMCfgValidator.KSPMMCfgValidator+<>c:<>9__1_1' (1) due to: Could not load file or assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies. assembly:/usr/local/bin/KSPMMCfgValidator.exe type:<>c member:(null)
   --- End of inner exception stack trace ---

Cause

Mysteries of .NET and Mono. The core error message Could not load file or assembly 'netstandard, Version=2.0.0.0 is essentially completely meaningless in terms of pointing toward a specific problem, so we have to fall back to trial and error.

Installing mono-complete fixed it, so I dug into its dependencies, and found that if I installed all of them except for mono-devel and mono-roslyn, the problem still happened, but if I installed those two packages, it was fixed. This should not happen; dev/compiler tools should not be needed to run an app that's already compiled, but that's where the evidence is leading us.

Changes

Now the Dockerfile installs mono-devel and mono-roslyn, which unfortunately pulls in most of the Mono cruft we were trying to trim in #18. But there appears to be no other way to get Mono to work here.

REPOSITORY                         TAG       IMAGE ID       CREATED             SIZE
kspckan/kspmmcfgvalidator          latest    b235ab1106a1   20 hours ago        197MB
cfgvalidator_test                  latest    ddda9bf1fa99   57 seconds ago      541MB

Fixes #19.