Makefile tweaks (#329)

* add --rm to build commands to prevent dangling images
relates to https://github.com/Kopano-dev/kopano-docker/pull/11
* remove login from makefile
* add docker login to travis instead
* readme tweaks

.travis.yml

@@ -30,6 +30,7 @@ deploy:
   - provider: script
     skip-cleanup: true
     script:
+      - echo "$docker_pwd" | docker login -u "$docker_login" --password-stdin
       - make publish
     on:
       branch: master