Add oxipng

Dockerfile

@@ -85,6 +85,12 @@ RUN curl http://apt.postgresql.org/pub/repos/apt/ACCC4CF8.asc | apt-key add - \
  #&& apt-get autoremove -y \
  && rm -rf /var/lib/apt/lists/*
 
+RUN mkdir /oxipng-install && cd /oxipng-install &&\
+      wget https://github.com/shssoichiro/oxipng/releases/download/v5.0.0/oxipng-5.0.0-x86_64-unknown-linux-musl.tar.gz &&\
+      tar -xzf oxipng-5.0.0-x86_64-unknown-linux-musl.tar.gz && cd oxipng-5.0.0-x86_64-unknown-linux-musl &&\
+      cp oxipng /usr/local/bin &&\
+      cd / && rm -rf /oxipng-install
+
 RUN cd /home/discourse/discourse/plugins \
  && for plugin in $(cat /tmp/install/plugin-list); do \
       git clone $plugin; \