Keep jcmd in JRE images to enable on demand flight recorder

[mattnelson]

The JRE images are purging[1] everything but the jre/bin directory. In order to enable flight recorder[2] on demand[3], jcmd should also be persisted.

[1] https://github.com/anapsix/docker-alpine-java/blob/8u172b11/8/172b11/server-jre/standard/Dockerfile#L40
[2] #6
[3] https://docs.oracle.com/javase/8/docs/technotes/guides/troubleshoot/tooldescr006.html

[anapsix]

jdk images do not have it removed
could that not be used to debug things?

[mattnelson]

We have switched back to the JDK image for now, but we would like to use the JRE image because of the size.