containers/dotnet: add initial container

[julieqiu]

Copied container from
https://github.com/jskeet/google-cloud-dotnet/tree/docker-generate/picard

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/

[jskeet]

Not actually reviewed - I'm assuming it's the same as on my repo. (Don't have opportunity to review anything really right now.)