Explicitely use Debian Buster in Dockerfile
Explicitely use Debian Buster in Dockerfile

Debian Bullseye was released two days ago, and is now the default
distribution for the Docker image.

Our image isn't compatible with Debian Bullseye right now, and we
haven't done any testing with it, so for now we're staying with Buster.

consul/consul,consul/consul,consul/consul,consul/consul,consul/consul