Makefile update.

[Johannestegner]

This pull request includes the following changes:

• The initial container creation in generate now uses a auto-removed container.
• Instead of using a local bound volume, a persistent docker volume is attached to the containers.
• example-keybaseca-volume was removed.
• Readme file was added to the docker folder to describe the main tasks in the make file.
• reset-permissions have been removed as it is no longer needed.
• create-volume have been introduced - it creates a new persistent volume in case one with the set name does not exist.
• clean now removes all containers using the volume and deletes the volume.
• Some minor changes to make the output easier to read was made.

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Observe!

This PR have a breaking change. The new volume does not import earlier created certificates. If one have those, the following could be used to import them into the volume (after generate).

docker run --rm -v $(pwd)/example-keybaseca-volume:/tmp -v  keybase-sshca-volume:/mnt cp /tmp/* /mnt

This should copy all the files from the old example-keybase-volume into the new persistent volume.

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notice:

I was unable to run the tests locally, so if that is possible, please let me know how to. I did run the make tasks a couple of hundred times and tried to cover each possible scenario, but that might not be enough, hehe.