Dockerfile: move elm to its own step

Dockerfile

@@ -1,21 +1,20 @@
-FROM xena/go:1.13.5 AS build
+FROM xena/go:1.13.5 AS go
 WORKDIR /mi
 COPY go.mod .
 COPY go.sum .
 RUN go mod download
 COPY . .
 RUN GOBIN=/mi/bin go install ./cmd/...
-RUN apk --no-cache add curl \
- && curl -L -o elm.gz https://github.com/elm/compiler/releases/download/0.19.1/binary-for-linux-64-bit.gz \
- && gunzip elm.gz \
- && chmod +x ./elm \
- && mv elm /usr/local/bin \
- && cd ./frontend \
- && elm make --output ../static/main.js src/Main.elm
+
+FROM xena/elm:0.19.1 AS elm
+WORKDIR /mi
+COPY ./frontend .
+RUN elm make --output main.js src/Main.elm
 
 FROM xena/alpine
-COPY --from=build /mi/bin /usr/local/bin
+COPY --from=go /mi/bin /usr/local/bin
 COPY ./run /run
-COPY --from=build /mi/static /mi/static
+COPY --from=go /mi/static /mi/static
+COPY --from=elm /mi/main.js /mi/static/main.js
 WORKDIR /mi
 CMD ["/bin/sh", "/run/start.sh"]