Add jdk-version Label and jdk check README

.gitignore

@@ -0,0 +1 @@
+/.idea/

Dockerfile

@@ -31,6 +31,8 @@ RUN apt-get update && apt-get install --no-install-recommends -y wget
 
 COPY --from=retrieve download.tar.gz /tmp/download.tar.gz
 
+LABEL jdk-version=11.0.8_10
+
 RUN mkdir -p /usr/lib/jvm \
  && tar xzf /tmp/download.tar.gz -C /usr/lib/jvm \
  && ln -s /usr/lib/jvm/* /usr/lib/jvm/jre \

README.md

@@ -1,3 +1,17 @@
 # openjdk-docker
 
 OpenJDK Docker image
+
+#### Retrieve the JDK version
+
+You can check the Java version bundled with a particular image tag, like this:
+
+```
+docker run -it --rm springcloud/openjdk:<image-tag> java -version
+```
+
+or by retrieving the `jdk-version` image label:
+
+```
+docker inspect springcloud/openjdk:<image-tag> | grep jdk-version
+```