Expose docker mount options

[clinta]

This PR adds support for configuring docker mount options that are available in docker API >= 1.25. These differ from the existing volume and volume_driver options in that they allow specifying a different driver per mount and allow specifying driver specific options for the volumes created.

https://docs.docker.com/engine/api/v1.25/#operation/ContainerCreate

[schmichael]

Hi @clinta!

Thanks for the PR; looks great. Being able to support binds and volume_drivers in the same task would be great. However, could you limit this PR to only add support for Volumes?

Binds are already supported through the volumes driver parameter and take into account relative vs absolute paths -- as well as a config flag to allow disabling them entirely to harden clients: https://www.nomadproject.io/docs/drivers/docker.html#volumes We'd either need to (1) support such logic in both places, (2) migrate the old parameter+behavior to the new API, or (3) just remove the bind options from this new code. (3) is the easiest, and we'll probably do (2) at some point down the road to drop support for the older less featureful API.

Generic tmpfs mount support is planned, so we'd rather not have it implemented multiple places/ways. This also allows properly accounting for the tmpfs's memory usage in scheduling decisions. In the mean time the secrets/ directory is a default tmpfs tasks can use to store small amounts of data like credentials.

Let me know if you have any questions or concerns!

[clinta]

How does this look? I've kept the type option in the structs, so that if you wish to make use of them in the future it's easy. But it now will default to volume and throw an error on any other type.

[schmichael]

Looks good! Thanks @clinta!

[schmichael]

@clinta Could you provide an example job file for documentation? I'll add it or feel free to open up a PR against our docs: https://github.com/hashicorp/nomad/blob/master/website/source/docs/drivers/docker.html.md

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