add docker cp completion for container ids

[bram2000]

- What I did
Modified the fish shell completion for docker cp such that it completes running container IDs as well as local file paths.

- How I did it
Added a new line to the competion file for fish, and edited a couple of the other cp completions to allow file paths.

- How to verify it

1. Run a docker container
2. Open fish shell
3. Type docker cp followed by TAB to initiate completion
4. Verify that file paths are completed, as well as the container ID of any running containers

- Description for the changelog
Update fish shell completion for docker cp to complete container IDs as well as file paths.

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