Run tests Docker in Docker (#261)

* Try running tests in Docker

* Spawn new container for each test

* Store test artifacts outside of mount

* When requested, build up to date image in test script

* sudo is unneccessary in containerized test env

* Skip azure test

* Backdate fixture file in JSON database

* Pin versions for azure tools

* Mount temp volume for /var/lib/docker to prevent dangling ones created by VOLUME instruction

* Fail backdating tests with message

* Add some documentation on test setup

* Cache images

* Run compose stacks with shortened default timeout

test/ignore/docker-compose.yml

@@ -11,5 +11,5 @@ services:
       BACKUP_CRON_EXPRESSION: 0 0 5 31 2 ?
       BACKUP_EXCLUDE_REGEXP: '\.(me|you)$$'
     volumes:
-      - ./local:/archive
+      - ${LOCAL_DIR:-./local}:/archive
       - ./sources:/backup/data:ro