Add a build props file to force root container users when people publish containers with this library

[baronfel]

As we found, Actions that are container-based must execute as root. Per the docs:

Docker actions must be run by the default Docker user (root). Do not use the USER instruction in your Dockerfile, because you won't be able to access the GITHUB_WORKSPACE directory. For more information, see "Variables" and USER reference in the Docker documentation.

We can help users enforce this invariant, while also giving them a chance to skip if they promise they know what they're doing.

A binlog shows:

the import

the property set

[IEvangelist]

@all-contributors please add @baronfel for code

[allcontributors[bot]]

@IEvangelist

I've put up a pull request to add @baronfel! :tada: