Add metadata to docker container

This adds three labels `pull-request`, `branch` and `subdomain` to the
preview container.
In this way we can inspect the labels to obtain these informations back
using:

```
docker inspect --format '{{ index .Config.Labels "pull-request" }}'
container_id
docker inspect --format '{{ index .Config.Labels "branch" }}'
container_id
docker inspect --format '{{ index .Config.Labels "subdomain" }}'
container_id
```

Note: to get the PR number I used the `$CIRCLE_PULL_REQUEST` that
contains the URL of the associated pull request and the bash's built-in
parameter substitution `${var##Pattern}`. The way this works is by
removing a prefix that greedily matches `*/`, which is what the `##`
operator does.

Author: ChristianRimondi

source.yml

@@ -120,7 +120,7 @@ jobs:
           name: Build Docker Image On Docker Server
           command: |
             source $CIRCLE_BUILD_NUM
-            ssh << parameters.user >>@<< parameters.server >> "cd $DIRECTORY && << parameters.fetch_seed_data_command >> && docker build -t $CONTAINER --build-arg SUBDOMAIN=$SUBDOMAIN --file=<< parameters.dockerfile >> ."
+            ssh << parameters.user >>@<< parameters.server >> "cd $DIRECTORY && << parameters.fetch_seed_data_command >> && docker build --label \"pull-request=${CIRCLE_PULL_REQUEST##*/}\" --label \"branch=$CIRCLE_BRANCH\" --label \"subdomain=$SUBDOMAIN\" -t $CONTAINER --build-arg SUBDOMAIN=$SUBDOMAIN --file=<< parameters.dockerfile >> ."
       - run:
           name: Run Docker Container On Docker Server
           command: |