Update Dockerfile adding back in public directory (Automattic#8603)

* Update Dockerfile adding back in `public` directory

See Automattic#8516

In the build I mistakenly prevented copying in the `public` directory
thinking that it was generated during the build process.

Actually it held our static assets and needed to be there. Images were
missing, for example.

The same Dockerfile has been updated again but this time I have not
prevented Docker from incorporating the `public` directory

* Split into three main layers

This should cache better than the monolith from earlier

* Move cleanup around to actually save on space

.dockerignore

@@ -1,2 +1,3 @@
 .git
 node_modules
+build

Dockerfile

@@ -1,49 +1,63 @@
-FROM	debian:wheezy
-
+FROM       node:6.9.0-slim
 MAINTAINER Automattic
 
-WORKDIR /calypso
-
-RUN     mkdir -p /tmp
-COPY    ./env-config.sh /tmp/
-RUN     bash /tmp/env-config.sh
-RUN     apt-get -y update && apt-get -y install \
-          wget \
-          git \
-          python \
-          make \
-          build-essential
-
-ENV NODE_VERSION 6.9.0
-
-RUN     wget https://nodejs.org/dist/v$NODE_VERSION/node-v$NODE_VERSION-linux-x64.tar.gz && \
-          tar -zxf node-v$NODE_VERSION-linux-x64.tar.gz -C /usr/local && \
-          ln -sf /usr/local/node-v$NODE_VERSION-linux-x64 /usr/local/node && \
-          ln -sf /usr/local/node/bin/npm /usr/local/bin/ && \
-          ln -sf /usr/local/node/bin/node /usr/local/bin/ && \
-          rm node-v$NODE_VERSION-linux-x64.tar.gz
-
-# npmrc is created by env-config.sh. For local testing, an empty one is generated
-RUN     touch /usr/local/etc/npmrc && \
-          mkdir /usr/local/node/etc && \
-          cp /usr/local/etc/npmrc /usr/local/node/etc/npmrc
-
-ENV     NODE_PATH /calypso/server:/calypso/client
-
-# Install base npm packages to take advantage of the docker cache
-COPY    ./package.json /calypso/package.json
-COPY    ./npm-shrinkwrap.json /calypso/npm-shrinkwrap.json
-# Sometimes "npm install" fails the first time when the cache is empty, so we retry once if it failed
-RUN     npm install --production || npm install --production
-
-COPY     . /calypso
-
-# Build javascript bundles for each environment and change ownership
-RUN     CALYPSO_ENV=wpcalypso make build-wpcalypso && \
-          CALYPSO_ENV=horizon make build-horizon && \
-          CALYPSO_ENV=stage make build-stage && \
-          CALYPSO_ENV=production make build-production && \
-          chown -R nobody /calypso
-
-USER    nobody
-CMD     NODE_ENV=production node build/bundle-$CALYPSO_ENV.js
+WORKDIR    /calypso
+
+
+ENV        NODE_PATH=/calypso/server:/calypso/client
+
+# Build a "base" layer
+#
+# This layer should never change unless env-config.sh
+# changes. For local development this should always
+# be an empty file and therefore this layer should
+# cache well.
+#
+# env-config.sh
+#   used by systems to overwrite some defaults
+#   such as the apt and npm mirrors
+COPY       ./env-config.sh /tmp/env-config.sh
+RUN        true \
+           && bash /tmp/env-config.sh \
+           && apt-get -y update \
+           && apt-get -y install \
+                 git \
+                 make \
+           && apt-get autoremove -y \
+           && rm -rf /tmp/* \
+           && rm -rf /var/tmp/* \
+           && rm -rf /usr/share/{doc,doc-base,info,locale,man}/* \
+           && rm -rf /var/lib/apt/lists/* \
+           && rm -rf /var/lib/dpkg/info/* \
+           && true
+
+# Build a "dependencies" layer
+#
+# This layer should include all required npm modules
+# and should only change as often as the dependencies
+# change. This layer should allow for final build times
+# to be limited only by the Calypso build speed.
+#
+# Sometimes "npm install" fails the first time when the
+# cache is empty, so we retry once if it failed
+COPY       ./package.json ./npm-shrinkwrap.json /calypso/
+RUN        true \
+           && npm install --production || npm install --production \
+           && rm -rf /root/.npm \
+           && true
+
+# Build the final layer
+#
+# This contains built environments of Calypso. It will
+# change any time any of the Calypso source-code changes.
+COPY       . /calypso/
+RUN        true \
+           && CALYPSO_ENV=wpcalypso make build-wpcalypso \
+           && CALYPSO_ENV=horizon make build-horizon \
+           && CALYPSO_ENV=stage make build-stage \
+           && CALYPSO_ENV=production make build-production \
+           && chown -R nobody /calypso \
+           && true
+
+USER       nobody
+CMD        NODE_ENV=production node build/bundle-$CALYPSO_ENV.js