Re-organize Dockerfile to improve incremental builds. (mastodon#10212)

- Always run apt update before any other apt command. (This fixes
  incremental builds failing if a remote package is updated.)
- Only copy dependency lists before installing dependencies. (This means
  editing code doesn't force all dependencies to be re-downloaded.)
- Delete cache in the same layer that it is created. (Otherwise,
  deleting cache *increases* the size of the image on non-squashed
  builds.)
- Move the installation of some static dependencies to *before* Mastodon
  code is imported to Docker.

Dockerfile

@@ -19,7 +19,8 @@ RUN	echo "Etc/UTC" > /etc/localtime && \
 
 # Install jemalloc
 ENV JE_VER="5.1.0"
-RUN apt -y install autoconf && \
+RUN apt update && \
+	apt -y install autoconf && \
 	cd ~ && \
 	wget https://github.com/jemalloc/jemalloc/archive/$JE_VER.tar.gz && \
 	tar xf $JE_VER.tar.gz && \
@@ -33,7 +34,8 @@ RUN apt -y install autoconf && \
 ENV RUBY_VER="2.6.1"
 ENV CPPFLAGS="-I/opt/jemalloc/include"
 ENV LDFLAGS="-L/opt/jemalloc/lib/"
-RUN apt -y install build-essential \
+RUN apt update && \
+	apt -y install build-essential \
 		bison libyaml-dev libgdbm-dev libreadline-dev \
 		libncurses5-dev libffi-dev zlib1g-dev libssl-dev && \
 	cd ~ && \
@@ -51,13 +53,14 @@ RUN apt -y install build-essential \
 ENV PATH="${PATH}:/opt/ruby/bin:/opt/node/bin"
 
 RUN npm install -g yarn && \
-	gem install bundler
+	gem install bundler && \
+	apt update && \
+	apt -y install git libicu-dev libidn11-dev \
+	libpq-dev libprotobuf-dev protobuf-compiler
 
-COPY . /opt/mastodon
+COPY Gemfile* package.json yarn.lock /opt/mastodon/
 
-RUN apt -y install git libicu-dev libidn11-dev \
-	libpq-dev libprotobuf-dev protobuf-compiler && \
-	cd /opt/mastodon && \
+RUN cd /opt/mastodon && \
 	bundle install -j$(nproc) --deployment --without development test && \
 	yarn install --pure-lockfile
 
@@ -83,21 +86,16 @@ RUN apt update && \
 	useradd -m -u $UID -g $GID -d /opt/mastodon mastodon && \
 	echo "mastodon:`head /dev/urandom | tr -dc A-Za-z0-9 | head -c 24 | mkpasswd -s -m sha-256`" | chpasswd
 
-# Copy over masto source from building and set permissions
-COPY --from=build-dep --chown=mastodon:mastodon /opt/mastodon /opt/mastodon
-
 # Install masto runtime deps
 RUN apt -y --no-install-recommends install \
 	  libssl1.1 libpq5 imagemagick ffmpeg \
 	  libicu60 libprotobuf10 libidn11 libyaml-0-2 \
 	  file ca-certificates tzdata libreadline7 && \
 	apt -y install gcc && \
 	ln -s /opt/mastodon /mastodon && \
-	gem install bundler
-
-# Clean up more dirs
-RUN rm -rf /var/cache && \
-	rm -rf /var/apt
+	gem install bundler && \
+	rm -rf /var/cache && \
+	rm -rf /var/lib/apt
 
 # Add tini
 ENV TINI_VERSION="0.18.0"
@@ -106,6 +104,10 @@ ADD https://github.com/krallin/tini/releases/download/v${TINI_VERSION}/tini /tin
 RUN echo "$TINI_SUM tini" | sha256sum -c -
 RUN chmod +x /tini
 
+# Copy over masto source, and dependencies from building, and set permissions
+COPY --chown=mastodon:mastodon . /opt/mastodon
+COPY --from=build-dep --chown=mastodon:mastodon /opt/mastodon /opt/mastodon
+
 # Run masto services in prod mode
 ENV RAILS_ENV="production"
 ENV NODE_ENV="production"