fix: pin containerd.io to compatible version for Docker 28.x

Docker 28.x vendors containerd v2.1.x internally. containerd.io >= 2.3
ships a shim that returns a protobuf BootstrapResult which the vendored
2.1.x code cannot parse, causing every container start to fail with:
  "failed to create TTRPC connection: unsupported protocol"

The role now:
- Detects the installed Docker and containerd.io versions
- If Docker < 29 and containerd.io >= 2.3, downgrades to the latest
  compatible 2.2.x from the Docker APT repository
- Holds the containerd.io package to prevent future auto-upgrades

This was triggered by containerd.io 2.2.6 → 2.3.3 auto-upgrading via
apt, which broke all CI runners simultaneously.

Generated with [Devin](https://devin.ai)

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

ansible/roles/gitea_runner/defaults/main.yml

@@ -66,6 +66,15 @@ gitea_runner_log_level: "info"
 gitea_runner_container_label: "gitea-runner=true"
 gitea_runner_file: ".runner"
 
+# Containerd version pinning — Docker 28.x vendors containerd v2.1.x internally.
+# containerd.io >= 2.3 ships a shim that returns a protobuf BootstrapResult which
+# Docker 28.x's vendored containerd code cannot parse, causing:
+#   "failed to create TTRPC connection: unsupported protocol: \b\x03\x12Yunix"
+# When Docker 29+ is installed (it vendors containerd 2.3+), this pin is not needed.
+# Set to "" to skip the compatibility check and allow any containerd.io version.
+gitea_runner_containerd_max_compatible_major: 2
+gitea_runner_containerd_max_compatible_minor: 2
+
 # Docker installation (for rootless dependencies)
 gitea_runner_docker_gpg_key_path: "/etc/apt/keyrings/docker.gpg"
 gitea_runner_docker_apt_arch: "{{ 'amd64' if ansible_facts['architecture'] == 'x86_64' else ansible_facts['architecture'] }}"

ansible/roles/gitea_runner/tasks/rootless_docker.yml

@@ -53,6 +53,72 @@
   register: gitea_runner_docker_install
   when: ansible_facts['os_family'] == 'Debian'
 
+# Docker 28.x vendors containerd v2.1.x. containerd.io >= 2.3 ships a shim that
+# returns a protobuf BootstrapResult the vendored 2.1.x code cannot parse, causing
+# "failed to create TTRPC connection: unsupported protocol" on every container start.
+# Detect the mismatch and downgrade containerd.io to the latest compatible 2.2.x.
+- name: Check installed Docker CE version (Debian/Ubuntu)
+  ansible.builtin.command: dpkg-query -W -f='${Version}' docker-ce
+  register: gitea_runner_docker_version_check
+  changed_when: false
+  when: ansible_facts['os_family'] == 'Debian'
+
+- name: Check installed containerd.io version (Debian/Ubuntu)
+  ansible.builtin.shell: "set -o pipefail; dpkg-query -W -f='${Version}' containerd.io 2>/dev/null | cut -d: -f2 | cut -d- -f1"
+  args:
+    executable: /bin/bash
+  register: gitea_runner_containerd_version_check
+  changed_when: false
+  when: ansible_facts['os_family'] == 'Debian'
+
+- name: Determine if containerd.io is incompatible with installed Docker
+  ansible.builtin.set_fact:
+    gitea_runner_containerd_needs_downgrade: >-
+      {{
+        gitea_runner_containerd_version_check.stdout.split('.')[0] | int > gitea_runner_containerd_max_compatible_major
+        or (
+          gitea_runner_containerd_version_check.stdout.split('.')[0] | int == gitea_runner_containerd_max_compatible_major
+          and gitea_runner_containerd_version_check.stdout.split('.')[1] | int > gitea_runner_containerd_max_compatible_minor
+        )
+      }}
+    gitea_runner_docker_major: "{{ gitea_runner_docker_version_check.stdout.split('.')[0] | default('0') | int }}"
+  when: ansible_facts['os_family'] == 'Debian'
+
+- name: Find latest compatible containerd.io version (Debian/Ubuntu)
+  ansible.builtin.shell: |
+    set -o pipefail
+    apt-cache madison containerd.io \
+      | awk -F'|' '{print $2}' \
+      | tr -d ' ' \
+      | grep -E '^{{ gitea_runner_containerd_max_compatible_major }}\.{{ gitea_runner_containerd_max_compatible_minor }}\.' \
+      | head -1
+  args:
+    executable: /bin/bash
+  register: gitea_runner_containerd_compatible_version
+  changed_when: false
+  when:
+    - ansible_facts['os_family'] == 'Debian'
+    - gitea_runner_containerd_needs_downgrade | default(false)
+    - gitea_runner_docker_major | int < 29
+
+- name: Downgrade containerd.io to compatible version (Debian/Ubuntu)
+  ansible.builtin.apt:
+    name: "containerd.io={{ gitea_runner_containerd_compatible_version.stdout }}"
+    state: present
+    allow_downgrades: true
+  register: gitea_runner_containerd_downgrade
+  when:
+    - ansible_facts['os_family'] == 'Debian'
+    - gitea_runner_containerd_needs_downgrade | default(false)
+    - gitea_runner_docker_major | int < 29
+    - gitea_runner_containerd_compatible_version.stdout | length > 0
+
+- name: Hold containerd.io package to prevent auto-upgrade (Debian/Ubuntu)
+  ansible.builtin.dpkg_selections:
+    name: containerd.io
+    selection: hold
+  when: ansible_facts['os_family'] == 'Debian'
+
 - name: Update pacman cache (Arch Linux)
   community.general.pacman:
     update_cache: true