“Could not resolve host” suggests a DNS issue, i.e. your server is unable to find the IP address for github.com.
==================== REBUILD LOG ====================
x86_64 arch detected.
Ensuring launcher is up to date
Fetching origin
Launcher is up-to-date
This suggests to me that your host is able to resolve and connect to github.com unless you have cloned the discourse_docker repository from somewhere else. You can confirm this with the following commands to list the remote for the repository:
cd /var/discourse
git remote -v
which should output:
origin https://github.com/discourse/discourse_docker.git (fetch)
origin https://github.com/discourse/discourse_docker.git (push)
If that is the case then the subsequent error failing to resolve github.com would imply that the docker container is unable to resolve it despite the host being able to, suggesting a problem with docker connectivity.