Update mount.go to allow relative file paths

[justyntemme]

- What I did
Fix for #1203

I resolved the mount argument 'source' to it's absolute filepath.
- How I did it
I used the golang library to resolve before the argument is tested for being absolute
- How to verify it
sudo ./docker run --mount type=bind,source=../e2e,target=/app nginx

sudo docker exec -it NGINX_CONTAINER /bin/bash
ls /app
- Description for the changelog

Added ability to use relative file paths for mount

- A picture of a cute animal (not mandatory but encouraged)

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "patch-1" git@github.com:justyntemme/cli.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[rumpl]

Done in #3469