Add small readme for docker image

README.docker.md

+# Docker Image
+
+## Building Locally
+
+```sh
+docker build -t vulnerable_bssl .
+docker run --rm vulnerable_bssl
+```
+
+## Pulling from registry
+
+```sh
+docker pull hub.cs.upb.de/snhebrok/vulnerable-bssl/vulnerable_bssl:master
+docker run --rm hub.cs.upb.de/snhebrok/vulnerable-bssl/vulnerable_bssl:master
+```