Purge build deps to shrink image from 2.32 GB to 1.76 GB (CircleCI-Public#92)

The space savings are for the 12.15 image, built locally before and
after this commit.

Combine the pg and pg_cron layers to avoid installing & purging
build-depends several times.

Gosu is used in entrypoint, so it is not purged.
locales is required by Pg at runtime, so it is not purged.

Author: stig

Dockerfile.template

@@ -20,11 +20,9 @@ ENV PGTAP_VERSION 1.2.0
 ENV PARTMAN_VERSION 4.7.2
 
 USER root
-RUN apt-get update && apt-get install -y --no-install-recommends \
-		clang \
+RUN BUILD_DEPS="clang \
 		dirmngr \
 		gnupg \
-		gosu \
 		libclang-dev \
 		libicu-dev \
 		libipc-run-perl \
@@ -40,11 +38,14 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 		libxslt1-dev \
 		llvm \
 		llvm-dev \
-		locales \
 		postgresql-server-dev-all \
 		python3-dev \
 		tcl-dev \
-		uuid-dev \
+		uuid-dev" && \
+	apt-get update && apt-get install -y --no-install-recommends \
+	gosu \
+	locales \
+	$BUILD_DEPS \
 	&& \
 	rm -rf /var/lib/apt/lists/* && \
 	localedef -i en_US -c -f UTF-8 -A /usr/share/locale/locale.alias en_US.UTF-8 && \
@@ -78,18 +79,17 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 	# we can change from world to world-bin in newer releases
 	make world && \
 	make install-world && \
-	cd .. && rm -rf postgresql-${PG_VER}
+	cd .. && rm -rf postgresql-${PG_VER} \
+	&& \
+	git clone --depth 1 https://github.com/citusdata/pg_cron.git /pg_cron && \
+	cd /pg_cron && make && PATH=$PATH make install && \
+	rm -rf /pg_cron && \
+	apt-get purge -y --auto-remove $BUILD_DEPS
 
 RUN curl -sSL "https://github.com/pgpartman/pg_partman/archive/v${PARTMAN_VERSION}.tar.gz" | tar -xz && \
 	cd pg_partman-${PARTMAN_VERSION} && make NO_BGW=1 install && \
 	cd .. && rm -rf pg_partman-${PARTMAN_VERSION}
 
-# clones pg_cron extension for local use, this is not available wihout installing
-# run this as a separate layer so it caches better
-RUN git clone --depth 1 https://github.com/citusdata/pg_cron.git /pg_cron && \
-	cd /pg_cron && make && PATH=$PATH make install && \
-	rm -rf /pg_cron
-
 # Install pgTAP & pg_prove utility.
 RUN git clone --depth 1 -b "v$PGTAP_VERSION" https://github.com/theory/pgtap.git /pgtap && \
 	cd /pgtap && make && make install && \