dsub fails with Docker image that uses non-root user

[mbookman]

When dsub runs it writes the user script/command to the /src directory.
When the Docker image is set to use a non-root user, this typically leads to a permission problem:

2017/03/13 22:06:26 E: command failed: mkdir: cannot create directory '/src': Permission denied
(exit status 1)

User scan work around this by adding the following to their Dockerfile:

RUN mkdir /src
RUN chmod 777 /src

However, I think we should fix this in dsub by writing to /tmp/src.

[mbookman]

This should be fixed by #35.
The user script is now written to /mnt/data/script.
A test was added for this: https://github.com/googlegenomics/dsub/blob/master/test/integration/e2e_non_root.sh