fix: separate apt update after adding Docker repo, use variable for repo string

The apt_repository update_cache option wasn't reliably picking up the
new Docker APT repo. Split into separate apt update step. Also moved
the long repo string to a default variable to satisfy yaml line-length
lint rule.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

ansible/roles/gitea-runner/defaults/main.yml

@@ -31,3 +31,7 @@ gitea_runner_file: ".runner"
 
 # Docker installation (for rootless dependencies)
 docker_gpg_key_path: "/etc/apt/keyrings/docker.gpg"
+docker_apt_repo: >-
+  deb [arch={{ ansible_facts['architecture'] }} signed-by={{ docker_gpg_key_path }}]
+  https://download.docker.com/linux/{{ ansible_facts['distribution'] | lower }}
+  {{ ansible_facts['distribution_release'] }} stable