Support Docker volumes.

Docker has a feature where it can import or export "volumes", or subtrees of
the filesystem which can be shared in different ways between Docker containers,
or between a container and the host machine.

This adds the command to the Dockerfile which sets up the knowledge inside
Docker. Also, if a directory is created which does not exist, it will be owned
by root. This would be unfortunate if, for example, the exported directory
was the logs/ directory for Play. So the directories are created beforehand.
Documentation for docker support.
Appropriate VOLUME and RUN lines should be created when volumes are exported.
Merge pull request #281 from garycoady/wip/docker-documentation

Documentation for docker support.
Merge pull request #280 from garycoady/wip/docker-volume

Support Docker volumes.
- Make the /var/log/<pkg> symlink doc match the package location.
- Now points to <install>/logs
Merge pull request #283 from paddymahoney/master

- Make the /var/log/<pkg> symlink doc match the package location.
Support building and publishing Docker images.

This provides reasonable behaviour for Docker publish/publishLocal tasks.
docker:publishLocal maps to "docker build -t [dockerTarget] ."
docker:publish maps to "docker push -t [dockerTarget]"
docker:dockerTarget has a default value of [dockerRepository]/[normalizedName]:[version]