Docker multi stage build for Lucky Framework app

Dockerfile

FROM crystallang/crystal:0.34.0-alpine-build AS build-env
ARG LUCKY_ROOT=/app
ARG DEV_PACKAGES="nodejs yarn"
ENV LUCKY_ENV=production
ENV NODE_ENV=production

WORKDIR $LUCKY_ROOT

# install packages
RUN apk update \
    && apk add --update --no-cache $DEV_PACKAGES

COPY shard* package.json yarn.lock ./

# shards, yarn, compile assets and crystal build are in different commands
# because docker will create different layers and cache it
# so when you build again, if nothing was changed, it will build blazing fast
# and in case only js or crystal code is changed, will run only its part
# install shards
COPY shard* $LUCKY_ROOT/
RUN shards

RUN yarn install --production=false
COPY . .
RUN yarn prod
RUN crystal build --release --static src/start_server.cr

# Remove folders not needed in resulting image
# rm does not result into smaller docker layer image
# I'm leaving as reference
# RUN rm -rf bin config db lib node_modules script spec src tasks

############### Build step done ###############
FROM alpine

ARG LUCKY_ROOT=/app
ENV LUCKY_ENV=production

WORKDIR $LUCKY_ROOT
COPY --from=build-env $LUCKY_ROOT/start_server $LUCKY_ROOT
COPY --from=build-env $LUCKY_ROOT/public $LUCKY_ROOT/public
EXPOSE 3000

CMD ["./start_server"]