Docker best practices

- Install and remove OS and Go build deps in a single layer
- Use COPY instead of ADD
- WORKDIR now resolves ENV variables, use one
- Use VOLUMES for configuration and datastore
- Added launcher script start-inbucket.sh
- Made docker-run.sh more powerful

etc/docker/defaults/start-inbucket.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+# start-inbucket.sh
+# description: start inbucket (runs within a docker container)
+
+CONF_SOURCE="$INBUCKET_HOME/defaults"
+CONF_TARGET="/con/configuration"
+
+set -eo pipefail
+
+install_default_config() {
+  local file="$1"
+  local source="$CONF_SOURCE/$file"
+  local target="$CONF_TARGET/$file"
+
+  if [ ! -e "$target" ]; then
+    echo "Installing default $file to $CONF_TARGET"
+    install "$source" "$target"
+  fi
+}
+
+install_default_config "inbucket.conf"
+install_default_config "greeting.html"
+
+exec "$INBUCKET_HOME/bin/inbucket" $*