create docker tag v1 to git tag name

Signed-off-by: Avelino <t@avelino.xxx>

.travis.yml

@@ -39,7 +39,7 @@ deploy:
     script:
       - docker build . -t prest/prest:v1
       - docker push prest/prest:v1
-      - docker push prest/prest:$TRAVIS_TAG
+      - docker tag prest/prest:v1 prest/prest:$TRAVIS_TAG && docker push prest/prest:$TRAVIS_TAG
       - curl -sL https://git.io/goreleaser | bash
     on:
       tags: true