we have to be able to install lua modules too

Dockerfile

@@ -21,13 +21,28 @@ FROM openresty/openresty:stretch
 # based on: https://git.occrp.org/libre/watchful-nginx
 MAINTAINER Michał "rysiek" Woźniak <rysiek@occrp.org>
 
-# we might need to install some packages, but doing this in the entrypoint doesn't make any sense
+# we might need to install some packages or Lua modules,
+# but doing this in the entrypoint doesn't make any sense
+ARG INSTALL_LUA_MODULES    
 ARG INSTALL_PACKAGES
-RUN export DEBIAN_FRONTEND=noninteractive && apt-get update && apt-get install -y \
+
+# packages install
+RUN export DEBIAN_FRONTEND=noninteractive; \
+    # if we are to install Lua modules later, we need luarocks
+    if [ "$INSTALL_LUA_MODULES" != "" ]; then \
+        export INSTALL_PACKAGES="$INSTALL_PACKAGES luarocks"; \
+    fi; \
+    # get on with the show
+    apt-get update && apt-get install -y \
         $INSTALL_PACKAGES ca-certificates inotify-tools gnupg2  \
         --no-install-recommends && \
     rm -rf /var/lib/apt/lists/*
-    
+
+# Lua modules install
+RUN for LUAROCK in $INSTALL_LUA_MODULES ; do \
+        luarocks install $LUAROCK; \
+    done;
+
 COPY run.sh /run.sh
 RUN chmod +x /run.sh