Include additional instructions in Dockerfiles

Allows the insertion of additional Dockerfile instructions

The :instructions value is taken from the :uberimage options map
in project.clj and specifies a list of Dockerfile instructions to
be inserted immediately after the FROM instruction at the start
of the Dockerfile.

Motivation was the shellshock vulnerability : I needed a way to 
do a dist-upgrade on the base-image and not to have to wait
for upstream base-images to be updated.