Allow layer author, comment, and created-by metadata to be set

[FenTiger]

This adds an optional metadata argument to buildLayer which allows the corresponding information strings to be set in the image history.

[FenTiger]

I think I've addressed all your points, but I had some trouble getting the tests to run, so I'm not certain that the new test actually passes.

Hopefully I'll get time this weekend to install podman and have a closer look locally.

[nlewo]

I think you can add a test such as the one added by this pr.

Also, you don't really need to install podman since you would get it transparently via nix run .#tests.your-test

[FenTiger]

Also, you don't really need to install podman since you would get it transparently via nix run .#tests.your-test

The failure I see is

$ nix run .#tests.metadata
warning: Ignoring setting 'auto-allocate-uids' because experimental feature 'auto-allocate-uids' is not enabled
warning: Ignoring setting 'impure-env' because experimental feature 'configurable-impure-env' is not enabled
Copy to podman image metadata:pc579z7vyng8yf04h9qwsfvw40xgcmb2
ERRO[0000] finding newgidmap: exec: "newgidmap": executable file not found in $PATH 
ERRO[0000] (Unable to determine exit status)            
Error during setresgid(0): Invalid argument
Error: command required for rootless mode with multiple IDs: exec: "newuidmap": executable file not found in $PATH
image list
Error: command required for rootless mode with multiple IDs: exec: "newuidmap": executable file not found in $PATH

Actual output:
Error: command required for rootless mode with multiple IDs: exec: "newuidmap": executable file not found in $PATH

Expected pattern:
test author

Error: test failed

Maybe it's not podman that's missing, but I need to install something to make it work.

[FenTiger]

Fixed. The test is working now.

[FenTiger]

Is there anything else I need to do to this PR?

[nlewo]

Nothing more is actually required. Tests passed locally. Thank you.