added docker-multistage build instructions

README.md

@@ -64,6 +64,57 @@ test
 
 This will keep the payload smaller and will also avoid any issues when compiling dependencies.
 
+### Multistage Docker Builds
+
+You can also leverage [docker multistage build](https://docs.docker.com/develop/develop-images/multistage-build/) and [bitwalker/alpine-elixir](https://github.com/bitwalker/alpine-elixir) to lower your image size significantly.
+
+An example is shown below:
+
+```dockerfile
+FROM bitwalker/alpine-elixir-phoenix:latest AS phx-builder
+
+# Set exposed ports
+ENV MIX_ENV=prod
+
+# Cache elixir deps
+ADD mix.exs mix.lock ./
+RUN mix do deps.get, deps.compile
+
+# Same with npm deps
+ADD assets/package.json assets/
+RUN cd assets && \
+    npm install
+
+ADD . .
+
+# Run frontend build, compile, and digest assets
+RUN cd assets/ && \
+    npm run deploy && \
+    cd - && \
+    mix do compile, phx.digest
+
+FROM bitwalker/alpine-elixir:latest
+
+EXPOSE 5000
+ENV PORT=5000 MIX_ENV=prod
+
+COPY --from=phx-builder /opt/app/_build /opt/app/_build
+COPY --from=phx-builder /opt/app/priv /opt/app/priv
+COPY --from=phx-builder /opt/app/config /opt/app/config
+COPY --from=phx-builder /opt/app/lib /opt/app/lib
+COPY --from=phx-builder /opt/app/deps /opt/app/deps
+COPY --from=phx-builder /opt/app/.mix /opt/app/.mix
+COPY --from=phx-builder /opt/app/mix.* /opt/app/
+
+# alternatively you can just copy the whole dir over with:
+# COPY --from=phx-builder /opt/app /opt/app
+# be warned, this will however copy over non-build files
+
+USER default
+
+CMD ["mix", "phx.server"]
+```
+
 ## License
 
 MIT