refactor Dockerfile to reduce number of layers

Dockerfile

@@ -4,23 +4,17 @@ MAINTAINER Dietrich Rordorf <[email protected]>
 
 USER root
 
-# exit on error
-RUN set -e
-
-# install ruby dependencies
-RUN apk add --no-cache ruby ruby-bigdecimal ruby-json libstdc++ sqlite-libs
-
-# install mailcatcher
-RUN apk add --no-cache --virtual .build-deps ruby-dev make g++ sqlite-dev
-RUN gem install mailcatcher --no-ri --no-rdoc
+# set exit on error flag, install ruby deps, build mailcatcher, remove buold deps
+RUN set -e && \
+    apk add --no-cache ruby ruby-bigdecimal ruby-json libstdc++ sqlite-libs && \
+    apk add --no-cache --virtual .build-deps ruby-dev make g++ sqlite-dev && \
+    gem install mailcatcher --no-ri --no-rdoc && \
+    apk del .build-deps && \
+    rm -rf /tmp/* /var/tmp/*
 
 # expose ports
 EXPOSE 1025
 EXPOSE 1080
 
-# remove no longer needed packages
-RUN apk del .build-deps
-RUN rm -rf /tmp/* /var/tmp/*
-
-# run process
+# entrypoint: run mailcatcher process
 CMD ["mailcatcher", "-f", "--ip=0.0.0.0"]