basic buildserver docker image
This builds a docker image using the buildserver provision scripts and includes the Vagrant standard setup so that it is a close as possible to the production buildserver image. This does not pre-install any Gradle, Android SDK, or NDK packages. This should hopefully work in CI, since gradle builds will auto-download the Android SDK packages it needs. Also gradlew-fdroid will download the gradle binaries and fdroid build will download the NDK binaries.
This should also make it easier to migrate the buildserver to Debian/bullseye since the changes here will also take effect in the image used for fdroiddata CI. !925 (closed) Ultimately, this Docker image setup should be fully synced with makebuildserver !1055.