Switch to crosbymichael/golang trusted base

Dockerfile

@@ -1,19 +1,8 @@
 # Docker build file for Inbucket, see https://www.docker.io/
 # Inbucket website: http://inbucket.org/
-FROM ubuntu:14.04
+FROM crosbymichael/golang
 MAINTAINER James Hillyerd, @jameshillyerd
 
-# To force the upgrade packages change REFRESHED_AT date, otherwise Docker
-# will cache the old updates
-ENV REFRESHED_AT 2014-05-19
-
-# Update Ubuntu
-RUN apt-get -q update \
-  && DEBIAN_FRONTEND=noninteractive apt-get -qy upgrade
-
-# Clean up APT when done.
-RUN apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
-
 # Install Inbucket
 ENV INBUCKET_HOME /opt/inbucket
 ADD inbucket $INBUCKET_HOME/inbucket
@@ -28,5 +17,7 @@ EXPOSE 25
 EXPOSE 80
 EXPOSE 110
 
-# Start Inbucket
-CMD $INBUCKET_HOME/inbucket $INBUCKET_HOME/inbucket.conf
+# Start Inbucket (WORKDIR doesn't support env vars)
+WORKDIR /opt/inbucket
+ENTRYPOINT ["./inbucket"]
+CMD ["inbucket.conf"]