use drone env for tagged version number

Dockerfile

@@ -12,6 +12,8 @@ ARG TRAVIS_PULL_REQUEST
 ARG TRAVIS_PULL_REQUEST_SHA
 ARG TRAVIS_REPO_SLUG
 ARG TRAVIS_TAG
+ARG DRONE_TAG
+ARG DRONE_COMMIT
 
 WORKDIR /go/src/github.com/umputun/remark
 
@@ -33,6 +35,10 @@ RUN if [ -z "$COVERALLS_TOKEN" ] ; then \
 
 RUN \
     version=$(git rev-parse --abbrev-ref HEAD)-$(git describe --abbrev=7 --always --tags)-$(date +%Y%m%d-%H:%M:%S) && \
+    if [ -z "$DRONE_TAG" ] ; then \
+    echo "runs outside of drone" \
+    else version=$DRONE_TAG-${DRONE_COMMIT:0:7}-$(date +%Y%m%d-%H:%M:%S); \ 
+    fi && \
     echo "version $version" && \  
     go build -o remark -ldflags "-X main.revision=${version} -s -w" ./app