add Dockerfile to test CI

docker/Dockerfile

@@ -0,0 +1,8 @@
+# Container image that runs your code
+FROM alpine:3.10
+
+# Copies your code file from your action repository to the filesystem path `/` of the container
+COPY docker/entrypoint.sh /entrypoint.sh
+
+# Code file to execute when the docker container starts up (`entrypoint.sh`)
+ENTRYPOINT ["/entrypoint.sh"]