Add chroot_bindings for exec driver to configure bind-mounts

[ashald]

Addresses #2115.

As ability to bind-mount parts of host FS while running under exec driver is essential for our organization to be able to adopt Nomad I decided to give it a shot. I've made a build from this branch and it runs without an issue on dozens of hosts while we continue our experiments with Nomad. Would be amazing if it could become a part of Nomad itself.

In terms of the implementation I think it's done although it'd be nice to add few tests. Also I'm not sure about styling - whether config option name is appropriate and so on so I decided to bounce this PR with Hashicorp and community. If it's something you don't mind to merge into the core I'd be glad to polish the PR unless you're happy with it or handle the sources to you (well, I already did that :) ) if you prefer to finish it on your own.

Please let me know what you think.

[chelseakomlo]

Hi @ashald, thanks for this PR.

Looking at the Travis build, it looks like there are a couple errors (see below). Do you mind fixing these up?

client/allocdir/task_dir_linux_test.go:28: td.unmountSpecialDirs undefined (type *TaskDir has no field or method unmountSpecialDirs)
client/allocdir/task_dir_linux_test.go:60: td.unmountSpecialDirs undefined (type *TaskDir has no field or method unmountSpecialDirs)
client/allocdir/task_dir_linux_test.go:69: td.unmountSpecialDirs undefined (type *TaskDir has no field or method unmountSpecialDirs)

[ashald]

@chelseakomlo I was told that there are plans for "platform-level" support for volumes planned in Nomad in future and therefore this PR doesn't make any sense as it will be fundamentally incompatible with how volumes will be managed in Nomad. Therefore I abandoned this patch and going to close the PR.

[prologic]

Would someone from Hashicorp mind answering the following?

Given that #5377 and #5378 were only created a week ago; Can I resurrect this old PR onto current master and get tests working as a short-term work-around for "lack of volumes" in Nomad? We are blocked by this missing feature and we have two choices a) Switch from exec to docker Execution Driver or b) Resurrect this PR (both of which are likely equally as much work for us)

[github-actions]

I'm going to lock this pull request because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active contributions.
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