Move roles/matrix* to roles/custom/matrix*

This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.

roles/custom/matrix-base/tasks/server_base/setup_redhat.yml

@@ -0,0 +1,31 @@
+---
+
+- name: Ensure Docker repository is enabled
+  ansible.builtin.template:
+    src: "{{ role_path }}/files/yum.repos.d/docker-ce-centos.repo"
+    dest: "/etc/yum.repos.d/docker-ce.repo"
+    owner: "root"
+    group: "root"
+    mode: 0644
+  when: matrix_docker_installation_enabled | bool and matrix_docker_package_name == 'docker-ce'
+
+- name: Ensure Docker's RPM key is trusted
+  ansible.builtin.rpm_key:
+    state: present
+    key: https://download.docker.com/linux/centos/gpg
+  when: matrix_docker_installation_enabled | bool and matrix_docker_package_name == 'docker-ce'
+
+- name: Ensure yum packages are installed
+  ansible.builtin.yum:
+    name:
+      - "{{ matrix_ntpd_package }}"
+    state: present
+    update_cache: true
+
+- name: Ensure Docker is installed
+  ansible.builtin.yum:
+    name:
+      - "{{ matrix_docker_package_name }}"
+      - docker-python
+    state: present
+  when: matrix_docker_installation_enabled | bool