ubuntu bootstrapping: modify docker cfg instead of full rewrite. (#395)

Adding ipv6 support for docker bootstrapping on ubuntu was rewriting
the full docker daemon.json config file.

This change just adds (or overwrites) the required fields to add ipv6
support.

This will allow switching docker storage to /mnt in our github ci
workflows right at the beginning on each job, instead of doing it right
after this script had rewritten.

config/vagrant/scripts/bootstrap-docker-ubuntu.sh

@@ -1,12 +1,9 @@
 #!/usr/bin/env bash
 
-# update docker config
-sudo bash -c 'cat <<- EOF > /etc/docker/daemon.json
-{
-  "ipv6": true,
-  "fixed-cidr-v6": "2001:db8:1::/64"
-}
-EOF'
+# update docker config to include ipv6 support
+jq '. +={"ipv6": true, "fixed-cidr-v6": "2001:db8:1::/64"}' /etc/docker/daemon.json > /tmp/new-docker-daemon.json
+sudo cp /tmp/new-docker-daemon.json /etc/docker/daemon.json
+rm /tmp/new-docker-daemon.json
 
 # restart docker
 sudo systemctl restart docker