Add support for multi-platform builds with docker buildx

- also added docs

docs/docker.md

@@ -0,0 +1,12 @@
+# Docker Setup
+
+## Build Image:
+`docker build -t tshock .`
+## and run:
+```bash
+docker run -p 7777:7777 -p 7878:7878 \
+           -v <save path>:/tshock \
+           -v <world path>:/worlds \
+           -v <plugin path>:/plugins \
+           --rm -it tshock [-world /worlds/<world file>] <other cmdline flags>
+```