travis: add deploy step to automatically push the container image

Configure .travis.yml to automatically push builds on the master branch
to the container image registry (quay.io by default). This will
automatically make the latest "experimental" version of NFD, built from
the tip of the master branch, available in the upstream Docker
repository.

IMAGE_REPO_USER and IMAGE_REPO_PASSWORD environment variables must be
defined in the travis repositorys settings in order for the deployment
step to be triggered.

.travis.yml

@@ -1,7 +1,19 @@
 language: minimal
 
+env:
+    global:
+        # Sanitize git branch name into a valid docker tag name
+        - IMAGE_TAG_NAME=$(echo -n $TRAVIS_BRANCH | tr -c "[a-zA-Z0-9._'" "_")
+
 services:
     - docker
 
 script:
-    - make image
+    - make image -e
+
+deploy:
+    on:
+        branch: master
+        condition: -n "$IMAGE_REPO_USER"
+    provider: script
+    script: echo "$IMAGE_REPO_PASSWORD" | docker login -u "$IMAGE_REPO_USER" --password-stdin quay.io && make push -e