Add option to run pre/post commands for any container (#73)

* Add option to run pre commands on arbitrary container

* Correctly handle quoted args in commands

* Provide defaults for test version arg

* Allow filtering of target containers

* Add documentation on exec commands

* Use mysqldump in exec test

* Add mysqldump section to recipes

* Also run commands test in swarm mode

* Use name instead of id

* Add syntax highlighting

* Add missing license headers

test/compose/docker-compose.yml

@@ -22,7 +22,7 @@ services:
       - webdav_backup_data:/var/lib/dav
 
   backup: &default_backup_service
-    image: offen/docker-volume-backup:${TEST_VERSION}
+    image: offen/docker-volume-backup:${TEST_VERSION:-canary}
     hostname: hostnametoken
     depends_on:
       - minio