Extend the file section to include an option of when to perform the file creation

[gareththered]

Issue

As briefly discussed in https://discuss.linuxcontainers.org/t/delay-in-mounting-files-in-oci/24972 a file created on a container using a file block, such as the following, is not created until after the container has started.

    content = file("./users.conf)
    target_path = "/etc/sftp/users.conf"
    create_directories = true
  }

If this file is mandatory for container boot, then booting fails.

Proposal

A possible solution would be to add an option to the file block to create the file early, for use by containers; or late, for use by VMs. If the provider can automatically detect whether it's building a container or VM, this could be automated and invisible to users?

@sergeypdev mentioned a similar issue in #91

[gwenya]

@stgraber could you assign this to me?

I'm thinking an option to select the behaviour makes sense, I'm not sure if it needs to be automatic though.
As a user I would prefer to not be surprised by the default behaviour being different between containers and VMs, and I wouldn't want to change the default behaviour of the file blocks from what it is now, especially since 1.0.0 was recently released.

So my favoured approach would be to add a flag that if set to true would create the file before container start, and just not allow it to be set to true for VMs.

I'm also thinking the idea of a "needs restart" field as you mentioned in the linked discussion would be useful in addition, so that applications which don't automatically reload their config when the file changes could then be automatically restarted as needed by terraform.