make build faster using buildlkit

- use annotation for buildkit
- cache yarn packages
- compile before copying assets to allow parallel build of phases

to use it, run
`DOCKER_BUILDKIT=1 docker image build -f docker/production/Dockerfile .`

docker/production/Dockerfile

@@ -1,11 +1,13 @@
+# syntax=docker/dockerfile:1.2
+
 # First build the application assets
 FROM node:16-alpine as assets
 
 RUN apk add --no-cache python3 build-base libwebp-tools bash imagemagick ncurses
 WORKDIR /build
 COPY js .
-RUN yarn install \
-    && yarn run build
+RUN --mount=type=cache,target=/root/.cache yarn install
+RUN yarn run build
 
 # Then, build the application binary
 FROM elixir:1.12-alpine AS builder
@@ -24,6 +26,9 @@ COPY config/config.exs config/prod.exs ./config/
 COPY config/docker.exs ./config/runtime.exs
 COPY rel ./rel
 COPY support ./support
+
+RUN mix compile
+
 COPY --from=assets ./priv/static ./priv/static
 
 RUN mix phx.digest \