Use pngcrush from package manager.

Dockerfile

@@ -48,6 +48,7 @@ RUN curl http://apt.postgresql.org/pub/repos/apt/ACCC4CF8.asc | apt-key add - \
       nodejs \
       optipng \
       jhead \
+      pngcrush \
       pngquant \
       postgresql-client-${PG_MAJOR} \
       postgresql-contrib-${PG_MAJOR} libpq-dev libreadline-dev \
@@ -66,7 +67,6 @@ RUN curl http://apt.postgresql.org/pub/repos/apt/ACCC4CF8.asc | apt-key add - \
  && /tmp/install/imagemagick \
  # Validate install
  && ruby -Eutf-8 -e "v = \`convert -version\`; %w{png tiff jpeg freetype}.each { |f| unless v.include?(f); STDERR.puts('no ' + f +  ' support in imagemagick'); exit(-1); end }" \
- && /tmp/install/pngcrush \
  && addgroup --gid 1000 discourse \
  && adduser --system --uid 1000 --ingroup discourse --shell /bin/bash discourse \
  && cd /home/discourse \

install/pngcrush

-#!/bin/bash -e
-cd /tmp
-wget https://sourceforge.net/projects/pmt/files/pngcrush/$PNGCRUSH_VERSION/pngcrush-$PNGCRUSH_VERSION.tar.gz/download -O pngcrush-$PNGCRUSH_VERSION.tar.gz
-tar zxf pngcrush-$PNGCRUSH_VERSION.tar.gz
-cd pngcrush-$PNGCRUSH_VERSION
-make && cp -f pngcrush /usr/local/bin
-cd /
-rm -fr /tmp/pngcrush-$PNGCRUSH_VERSION
-rm /tmp/pngcrush-$PNGCRUSH_VERSION.tar.gz