Add a --readonly flag to allow the container's root filesystem to be
mounted as readonly. This can be used in combination with volumes to
force a container's process to only write to locations that will be
persisted. This is useful in many cases where the admin controls where
they would like developers to write files and error on any other
locations.
Closes#7923Closes#8752
Signed-off-by: Michael Crosby <crosbymichael@gmail.com>