Add org.opencontainers.image.source label

Signed-off-by: Riddhesh Sanghvi <riddhesh237@gmail.com>

Author: mrrobot47

Dockerfile

@@ -4,7 +4,7 @@ LABEL "com.github.actions.icon"="bell"
 LABEL "com.github.actions.color"="yellow"
 LABEL "com.github.actions.name"="Slack Notify"
 LABEL "com.github.actions.description"="This action will send notification to Slack"
-
+LABEL "org.opencontainers.image.source"="https://github.com/rtCamp/action-slack-notify"
 
 WORKDIR ${GOPATH}/src/github.com/rtcamp/action-slack-notify
 COPY main.go ${GOPATH}/src/github.com/rtcamp/action-slack-notify