Automate release of base image when new Brave version is released.

[ikreymer]

Since Brave publishes releases to GitHub, we should be able to automate pushing a new base image here whenever a new stable version of Brave is released.
Perhaps can check with a GH action daily, or even more frequently, and bump the version in brave-version.txt?

Later, can also look at releasing a new version of browsertrix-crawler when a new base image is built as well.