PR for CopyFileFromContainer

[codepitbull]

I wasn't sure how you track this repo so I filed this.

I opened a PR (#347) that adds the capability to copy a file from a container.
I built this because I am working with a k3s-container and I needed to get the kubeconfig from it.
Since this is a functionality available in testcontainers-java I thought it would be a nice addition for testcontainser-go.

Feel free to request any changes you feel are needed to get this into the main-branch :)

Also: Thanks a lot for this project, it helped us quite a bit here at Instana.

[codepitbull]

Thanks for merging the PR, I think this solves this issue :)