It would be easier if you could manage your credential in git-credential-manager on your macOS/Windows host but then have those credentials automatically used by git inside your dev container.

That's what this utility does. git-credential-forwarder sets up a proxy server on your host that uses the git credential interface to pass credentials back and forth between host and container. Nothing ever gets stored in your container, git (in the container) just requests credentials via the proxy (on the host) when it needs them.

This is the same trick VS Code uses to achieve this outcome. Only this is open source and doesn't require you to be using VS Code.