Clear default Dockerfile assignments for *PLATFORM

These kept Docker buildx from automatically building for multiple
platforms.

Dockerfile

@@ -1,10 +1,8 @@
-ARG TARGETPLATFORM=linux/amd64
-ARG BUILDPLATFORM=${TARGETPLATFORM}
+# TARGETPLATFORM and BUILDPLATFORM are automatically filled in by Docker buildx.
+# They should not be set in the global scope manually.
 
 FROM --platform=${BUILDPLATFORM} mcr.microsoft.com/dotnet/sdk:6.0 AS builder
 
-ARG TARGETPLATFORM
-
 # Copy build context
 WORKDIR /TShock
 COPY . ./
@@ -12,6 +10,10 @@ COPY . ./
 # Build and package release based on target architecture
 RUN dotnet build -v m
 WORKDIR /TShock/TShockLauncher
+
+# Make TARGETPLATFORM available to the container.
+ARG TARGETPLATFORM
+
 RUN \ 
   case "${TARGETPLATFORM}" in \
     "linux/amd64") export ARCH="linux-x64" \

docs/docker.md

@@ -14,15 +14,15 @@ Open ports can also be passed through using `-p <host_port>:<container_port>`.
 
 For Example:
 ```bash
-# Building the image
-docker build -t tshock:linux-amd64 --build-arg TARGETPLATFORM=linux/amd64 .
+# Building the image using buildx and loading it into docker
+docker buildx build -t tshock:latest --load .
 
 # Running the image
 docker run -p 7777:7777 -p 7878:7878 \
            -v /home/cider/tshock/:/tshock \
            -v /home/cider/.local/share/Terraria/Worlds:/worlds \
            -v /home/cider/tshock/plugins:/plugins \
-           --rm -it tshock:linux-amd64 \
+           --rm -it tshock:latest \
            -world /worlds/backflip.wld -motd "OMFG DOCKER"
 ```
 
@@ -33,7 +33,7 @@ Using `docker buildx`, you could build [multi-platform images](https://docs.dock
 For Example:
 ```bash
 # Building the image using buildx and loading it into docker
-sudo docker buildx build -t tshock:linux-arm64 --platform linux/arm64 --load .
+docker buildx build -t tshock:linux-arm64 --platform linux/arm64 --load .
 
 # Running the image
 docker run -p 7777:7777 -p 7878:7878 \