Build and push new tagged releases to Docker Hub

Based on method described in https://sebest.github.io/post/using-travis-ci-to-build-docker-images/

.travis.yml

@@ -40,6 +40,11 @@ jobs:
         - export TAG=`echo $TRAVIS_TAG | sed 's/\./-/g' | sed 's/.*/v&/'`
         - echo "{\"name\":\"datasette-latest-$ALIAS\",\"alias\":\"$TAG.datasette.io\"}" > now.json
         - now alias --token=$NOW_TOKEN
+        # Build and release to Docker Hub
+        - docker login -e $DOCKER_EMAIL -u $DOCKER_USER -p $DOCKER_PASS
+        - export REPO=datasetteproject/datasette
+        - docker build -f Dockerfile -t $REPO:$TRAVIS_TAG .
+        - docker push $REPO
       deploy:
         - provider: pypi
           user: simonw