Add skip_cleanup to Docker deployment

Author: fwcd

.travis.yml

@@ -46,3 +46,4 @@ jobs:
             - docker login docker.pkg.github.com -u fwcd -p "$GH_TOKEN"
             - docker build -t docker.pkg.github.com/fwcd/kotlin-language-server:latest .
             - docker push fwcd/kotlin-language-server:latest
+          skip_cleanup: true