Remove container_id from env:

.travis.yml

@@ -1,26 +1,22 @@
 sudo: required
 
 env:
-  - distribution: centos
-    version: 6
-    init: /sbin/init
-    run_opts: ""
-    container_id: $(mktemp)
+  - distribution: centos  # Linux distribution
+    version: 6            # Distribution version
+    init: /sbin/init      # Path to init executable (differs for SysVInit/Systemd)
+    run_opts: ""          # Additional options for running the Docker container
   - distribution: centos
     version: 7
     init: /usr/lib/systemd/systemd
     run_opts: "--privileged --volume=/sys/fs/cgroup:/sys/fs/cgroup:ro"
-    container_id: $(mktemp)
   - distribution: ubuntu
     version: 14.04
     init: /sbin/init
     run_opts: ""
-    container_id: $(mktemp)
   - distribution: ubuntu
     version: 12.04
     init: /sbin/init
     run_opts: ""
-    container_id: $(mktemp)
 
 services:
   - docker
@@ -37,6 +33,7 @@ script:
   # Run test playbook
   #
 
+  - container_id=$(mktemp)
     # Run container in detached state
   - sudo docker run --detach --volume="${PWD}":/etc/ansible/roles/role_under_test:ro ${run_opts} ${distribution}-${version}:ansible "${init}" > "${container_id}"