flatten the "playbooks" directory for better structure

Because of how Ansible local relative search paths work, the global
"files" and "templates" directories need to be next to the playbooks.
However its not intuitive to look into the playbooks directory to find
the files and templates for a host.
Therefore flatten the playbooks directory to get rid of this confusing
structure.

Also see:
https://docs.ansible.com/ansible/latest/playbook_guide/playbook_pathing.html#resolving-local-relative-paths

roles/docker/README.md

@@ -0,0 +1,27 @@
+# Role `docker`
+
+Makes sure Docker Engine and other related packages are installed from the Docker repos on the specified hosts.
+For details see: [`tasks/main/02_docker_install.yaml`](./tasks/main/02_docker_install.yaml).
+
+## Supported Distributions
+
+The following distributions are supported:
+
+- Debian 11
+
+## Required Arguments
+
+None.
+
+## Updates
+
+This role doesn't handle updates.
+However it uses the system package manager for installing Docker Engine and the other related packages, so when you're making sure the system packages are up-to-date, you're handling updates for the packages installed by this role as well.
+
+## `hosts`
+
+The `hosts` for this role need to be the machines for which you want to make sure Docker Engine and other related packages are installed from the Docker repos.
+
+## Links & Resources
+
+- <https://docs.docker.com/engine/install/debian/>

roles/docker/meta/main.yaml

@@ -0,0 +1,9 @@
+---
+dependencies:
+  - role: distribution_check
+    vars:
+      distribution_check__distribution_support_spec:
+        - name: Debian
+          major_versions:
+            - 11
+            - 12

roles/docker/tasks/main.yaml

@@ -0,0 +1,7 @@
+- name: make sure the Docker repo is setup
+  ansible.builtin.import_tasks:
+    file: main/01_repo_setup.yaml
+
+- name: make sure Docker Engine and other related packages are installed
+  ansible.builtin.import_tasks:
+    file: main/02_docker_install.yaml

roles/docker/tasks/main/01_repo_setup.yaml

@@ -0,0 +1,15 @@
+- name: make sure Dockers GPG key is added
+  ansible.builtin.get_url:
+    url: https://download.docker.com/linux/debian/gpg
+    dest: /etc/apt/trusted.gpg.d/docker.asc
+    mode: "0644"
+    owner: root
+    group: root
+  become: true
+
+- name: make sure Dockers APT repository is added
+  ansible.builtin.apt_repository:
+    repo: "deb [arch=amd64 signed-by=/etc/apt/trusted.gpg.d/docker.asc] https://download.docker.com/linux/debian {{ ansible_distribution_release }} stable"
+    filename: docker
+    state: present
+  become: true

roles/docker/tasks/main/02_docker_install.yaml

@@ -0,0 +1,11 @@
+- name: make sure Docker Engine and other related packages are installed
+  ansible.builtin.apt:
+    name:
+      - docker-ce
+      - docker-ce-cli
+      - containerd.io
+      - docker-buildx-plugin
+      - docker-compose-plugin
+    state: present
+    update_cache: true
+  become: true