Building docker image with JMXTRANS_VERSION doesn't affect the version built

[gangefors]

Using JMXTRANS_VERSION as described in the README will not build the version specified.

In the Dockerfile we find that this variable is always overwritten with the latest release from maven.org.

The only indication that the docker image doesn't contain the version specified is a warning message at the end of the build. This is easy to overlook. [Warning] One or more build-args [JMXTRANS_VERSION] were not consumed