Fix error: multiple definition of `disctype';

[aphex3k]

Fix error

multiple definition of `disctype'; CMakeFiles/friidumplibdir/disc.c.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status

Fixes #10

[aphex3k]

Also adds a fairly minimal Dockerfile to build and run friidump.

[ColleagueRiley]

You could also just make the uisctype variable static instead of just removing it. I think that matches what the original author was trying to go for more. Although I don't see much of a use of it being there myself, I've not looked much into the code.