Use cat instead of read<file

When reading a file, it is expected that
the read process will exit 1 as this is
expected whenever the input reaches an EOF.

Because it is not clear if the 'exit 1' is
from a successful read or a more serious error,
and as this edge-case of 'read' is not well-known,
we instead change this code to read the file using
'cat'.

The new code is moved into a function, is_docker_running.

Furthermore, we now quote the variables and check
for the existance of the pid file for better and safer
error handling.

Change-Id: Idb56b87349a5a84d5d255715cfb7191341363118
Closes-Bug: 1286441
Merge "Use cat instead of read<file"