feat: add dart example to custom container docs (#607)

Author: HomelessDinosaur

src/pages/reference/custom-containers.mdx

@@ -171,6 +171,35 @@ RUN apk update && \
 ENTRYPOINT ["/bin/main"]
 ```
 
+```docker {{ title: "Dart" }}
+FROM dart:stable AS build
+
+ARG HANDLER
+
+WORKDIR /app/
+
+# Resolve app dependencies.
+COPY pubspec.* ./
+RUN dart pub get
+
+# Ensure the ./bin folder exists
+RUN mkdir -p ./bin
+
+# Copy app source code and AOT compile it.
+COPY . .
+
+# Ensure packages are still up-to-date if anything has changed
+RUN dart pub get --offline
+RUN dart compile exe ./${HANDLER} -o bin/main
+
+FROM alpine
+
+COPY --from=build /runtime/ /
+COPY --from=build /app/bin/main /app/bin/
+
+ENTRYPOINT ["/app/bin/main"]
+```
+
 </CodeGroup>
 
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