Allow mount_path to be specified when defining volumes.

[TarekkMA]

The previous implementation of this feature, would result in the host_path and the mount_path to be the same with not option to make a custom mount path.

[Alexhuszagh]

Ah I can see why this would be needed. Say I need to include data while compiling from $HOME/.config/mydata, and since the home directory will be different paths for the host and mount, then you want to be able to mount this to the same logical point in the container? The logic has changed significantly in our code, so this will likely need a fresh PR, but I'm not opposed to this on principal.

Say, for example:

fn main() {
    let my_str = include_str!(concat!(env!("HOME"), "/path/to/file"));
    print!("{my_str}");
}

Your source code wouldn't compile unless FILE is set and to the proper path.

However...

I still think there's a better way of doing this? Environment variables aren't shared between the host and container, unless passthrough is used. And I don't think including data at compile time from an environment variable is that good an idea, but I mean it could be required?

[Alexhuszagh]

Actually closing since you can do this via DOCKER_OPTS, and for very esoteric behavior like this that would require much more than "zero" setup, since it is almost certainly host-specific, you can always provide it via another config option, and isn't a valid path which is what we planned to support.