Add support for multi-platform builds with docker buildx

- also added docs

Dockerfile

@@ -1,32 +1,32 @@
+FROM --platform=${BUILDPLATFORM} mcr.microsoft.com/dotnet/sdk:6.0 AS builder
 
-# Docker Instructions
-# Build Image:
-#  docker build -t tshock .
-# and run:
-#  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> <flags>
-
-FROM mcr.microsoft.com/dotnet/sdk:6.0 AS builder
-
-ARG ARCH=linux-x64
+ARG TARGETPLATFORM
 
 # Copy build context
 WORKDIR /TShock
 COPY . ./
 
-# Build and package release
-RUN dotnet build
+# Build and package release based on target architecture
+RUN dotnet build -v m
 WORKDIR /TShock/TShockLauncher
-RUN dotnet publish -o output/ -r ${ARCH} -f net6.0 -c Release -p:PublishSingleFile=true --self-contained false
+RUN case "${TARGETPLATFORM}" in \
+   "linux/amd64") export ARCH="linux-x64" \
+   ;; \
+   "linux/arm64") export ARCH="linux-arm64" \
+   ;; \
+   "linux/arm/v7") export ARCH="linux-arm" \
+   ;; \
+   "windows/amd64") export ARCH="win-x64" \
+   ;; \
+   *) echo "Error: Unsupported platform ${TARGETPLATFORM}" && exit 1 \
+   ;; \
+  esac && \
+  dotnet publish -o output/ -r "${ARCH}" -v m -f net6.0 -c Release -p:PublishSingleFile=true --self-contained false
 
 # Runtime image
-FROM mcr.microsoft.com/dotnet/runtime:6.0 AS runner
+FROM --platform=${TARGETPLATFORM} mcr.microsoft.com/dotnet/runtime:6.0 AS runner
 WORKDIR /server
 COPY --from=builder /TShock/TShockLauncher/output ./
-RUN mkdir -p /tshock /worlds /plugins
 
 VOLUME ["/tshock", "/worlds", "/plugins"]
 EXPOSE 7777 7878
@@ -38,4 +38,4 @@ ENTRYPOINT [ \
   "-crashdir", "/tshock/crashes", \
   "-worldselectpath", "/worlds", \
   "-additionalplugins", "/plugins" \
-]
+]