split KVM and docker configs

Config cannot have conflicting executor configurations. One _must_
specify the selected executor in a sub-section. eg. "[runners.docker]",
otherwise you end up with mysterious errors like:

 WARNING: Job failed: panic: runtime error: invalid memory address or
 nil pointer dereference

roles/gitlab-runner/tasks/main.yml

@@ -55,10 +55,17 @@
   with_items: [config, prepare, run, cleanup]
   when: ansible_local.gitlab_runner.executor == "kvm"
 
-- name: Install configuration for the gitlab runner
+- name: Install KVM configuration for the gitlab runner
   template:
-    src: gitlab-runner.toml
+    src: gitlab-runner-kvm.toml
     dest: /etc/gitlab-runner/config.toml
+  when: ansible_local.gitlab_runner.executor == "kvm"
+
+- name: Install Docker configuration for the gitlab runner
+  template:
+    src: gitlab-runner-docker.toml
+    dest: /etc/gitlab-runner/config.toml
+  when: ansible_local.gitlab_runner.executor == "docker"
 
 - name: Put gitlab-runner in the docker group
   user:

roles/gitlab-runner/templates/gitlab-runner-docker.toml

@@ -0,0 +1,18 @@
+concurrent = 5
+check_interval = 0
+log_level = "info"
+
+[session_server]
+  session_timeout = 1800
+
+[[runners]]
+  name = "{{ ansible_fqdn }}"
+  url = "https://{{ ansible_local.gitlab_runner.gitlab_domain }}"
+  token = "{{ ansible_local.gitlab_runner.runner_token }}"
+  executor = "{{ ansible_local.gitlab_runner.executor }}"
+
+  [runners.docker]
+    image = "debian:stable"
+    privileged = false
+    disable_cache = false
+    shm_size = 0

roles/gitlab-runner/templates/gitlab-runner-kvm.toml

@@ -1,5 +1,6 @@
 concurrent = 5
 check_interval = 0
+log_level = "info"
 
 [session_server]
   session_timeout = 1800