added '--rm' to docker command to clean up container after use

[seanreid-toast]

I didn't see anything that indicated I needed to manually up the version, but if there is, I'll do that before merging.

This patch adds --rm to the arguments passed to Docker when buf is invoked. This cleans up the list of stopped containers on a system.

For testing, I ran the tests included in ./gradle test and watched as containers were created to run buf and were immediately deleted after they terminated. All tests still passed.

[andrewparmet]

This project is versioned using the semantic versioning plugin.

[andrewparmet]

Published in version 0.3.2.