Persist the old preview URLs for future jobs

This uses the labels added in #4 to generate and persist a file with
a list of preview URLs associated with the containers removed during the
current orb execution.

This adopts the same approach used in #3, so the way of retrieving the
persisted file is the same:

```
  steps:
    - attach_workspace:
        # Must be absolute path or relative path from working_directory
        at: /tmp/workspace

    - run: |
        cat /tmp/workspace/old-preview-urls
```

Author: ChristianRimondi

source.yml

@@ -145,9 +145,11 @@ jobs:
             set +e
             ssh << parameters.user >>@<< parameters.server >> "docker ps | grep $CIRCLE_PROJECT_REPONAME | head -n << parameters.fresh_containers_count >>" | awk '{ print $1 }' | tr '\n' '|' | sed 's/|$/\n/' > grep_regex
             ssh << parameters.user >>@<< parameters.server >> "docker ps | grep $CIRCLE_PROJECT_REPONAME | grep -vE '`cat grep_regex`'" | awk '{print $2}' > old_containers
+            ssh << parameters.user >>@<< parameters.server >> "docker inspect --format '{{ index .Config.Labels \"subdomain\" }}' "`cat old_containers`"" | sed -r "s/(.*)/https:\/\/\1\.<< parameters.domain >>/g" > workspace/old-preview-urls
             ssh << parameters.user >>@<< parameters.server >> "docker rm -f "`cat old_containers`""
             ssh << parameters.user >>@<< parameters.server >> "rm -fr "`cat old_containers | sed "s/$CIRCLE_PROJECT_REPONAME-/$CIRCLE_PROJECT_REPONAME\//g"`""
       - persist_to_workspace:
           root: workspace
           paths:
             - preview-url
+            - old-preview-urls