chore: Docker login.

.travis.yml

@@ -4,7 +4,8 @@ go:
   - 1.11.x
   - 1.x
 
-sudo: false
+services:
+  - docker
 
 env:
   - GO111MODULE=on
@@ -23,6 +24,13 @@ install:
   - echo "TRAVIS_GO_VERSION=$TRAVIS_GO_VERSION"
   - go mod download
 
+before_deploy:
+  - >
+    if ! [ "$BEFORE_DEPLOY_RUN" ]; then
+      export BEFORE_DEPLOY_RUN=1;
+      echo "${DOCKER_PASSWORD}" | docker login -u "${DOCKER_USERNAME}" --password-stdin
+    fi
+
 deploy:
   - provider: script
     skip_cleanup: true