Make dockerfile more platform-agnostic

DiscordChatExporter.Cli.dockerfile

@@ -1,7 +1,16 @@
 # -- Build
-# Use the same platform for the build image as the runtime image, so that it's easier
-# to produce a self-contained assembly for the right platform.
-FROM mcr.microsoft.com/dotnet/sdk:7.0-runtime AS build
+# Specify the platform here so that we pull the SDK image matching the host platform,
+# instead of the target platform specified during build by the `--platform` option.
+# Use the .NET 8.0 preview because the `--arch` option is only available in that version.
+# https://github.com/dotnet/dotnet-docker/issues/4388#issuecomment-1459038566
+# TODO: switch images to Alpine once .NET 8.0 is released.
+# Currently, the correct preview version is only available on Debian.
+# https://github.com/dotnet/dotnet-docker/blob/main/samples/selecting-tags.md
+FROM --platform=$BUILDPLATFORM mcr.microsoft.com/dotnet/sdk:8.0-preview AS build
+
+# Expose the target architecture set by the `docker build --platform` option, so that
+# we can build the assembly for the correct platform.
+ARG TARGETARCH
 
 WORKDIR /build
 
@@ -13,33 +22,24 @@ COPY DiscordChatExporter.Cli ./DiscordChatExporter.Cli
 
 # Publish a self-contained assembly so we can use a slimmer runtime image
 RUN dotnet publish DiscordChatExporter.Cli \
+    --configuration Release \
     --self-contained \
     --use-current-runtime \
-    --configuration Release \
+    --arch $TARGETARCH \
     --output ./publish
 
 # -- Run
-# Use Alpine for the runtime image due to its smaller size.
-# Use `runtime-deps` instead of `runtime` since we're running a self-contained assembly.
-# https://github.com/dotnet/dotnet-docker/blob/main/samples/selecting-tags.md
-FROM mcr.microsoft.com/dotnet/runtime-deps:7.0-alpine
-
-# Alpine image doesn't include timezone data, which is needed for certain date/time operations
-# https://github.com/dotnet/dotnet-docker/blob/main/samples/enable-globalization.md
-RUN apk add --no-cache tzdata
+# Use `runtime-deps` instead of `runtime` because we have a self-contained assembly
+FROM --platform=$TARGETPLATFORM mcr.microsoft.com/dotnet/runtime-deps:7.0 AS run
 
 # Create a non-root user to run the app, so that the output files can be accessed by the host
 # https://github.com/Tyrrrz/DiscordChatExporter/issues/851
 RUN adduser --disabled-password --no-create-home dce
 USER dce
 
-COPY --from=build /build/publish /opt/dce
-
-# Add the app directory to the PATH so that it's easier to run the app via `docker exec`
-ENV PATH="$PATH:/opt/dce"
-
-# This directory is exposed to the user for mounting purposes, so it's important
-# that it stays the same for backwards compatibility.
+# This directory is exposed to the user for mounting purposes, so it's important that it always
+# stays the same for backwards compatibility.
 WORKDIR /out
 
-ENTRYPOINT ["DiscordChatExporter.Cli"]
+COPY --from=build /build/publish /opt/dce
+ENTRYPOINT ["/opt/dce/DiscordChatExporter.Cli"]