Added functionality to ease the use of X image

Functionality borrowed from the Raspberry Pi cross compile docker build
file from https://github.com/sdt/docker-raspberry-pi-cross-compiler

imagefiles/entrypoint.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+
+# This is the entrypoint script for the dockerfile. Executed in the
+# container at runtime.
+
+if [[ $# == 0 ]]; then
+    # Presumably the image has been run directly, so help the user get started.
+    cat /dockcross/dockcross
+    exit 0
+fi
+
+# If we are running docker natively, we want to create a user in the container
+# with the same UID and GID as the user on the host machine, so that any files
+# created are owned by that user. Without this they are all owned by root.
+# If we are running from boot2docker, this is not necessary.
+# The dockcross script sets the BUILDER_UID and BUILDER_GID vars.
+if [[ -n $BUILDER_UID ]] && [[ -n $BUILDER_GID ]]; then
+
+    BUILDER_USER=dockcross-user
+    BUILDER_GROUP=dockcross-group
+
+    groupadd -o -g $BUILDER_GID $BUILDER_GROUP 2> /dev/null
+    useradd -o -g $BUILDER_GID -u $BUILDER_UID $BUILDER_USER 2> /dev/null
+
+    # Run the command as the specified user/group.
+    exec chpst -u :$BUILDER_UID:$BUILDER_GID "$@"
+else
+    # Just run the command as root.
+    exec "$@"
+fi