Change base image of example Dockerfile

[gabrielgiussi]

As discussed in #15, the resulting image of the example Dockerfile may be too heavy (1.29 GB).
Also, the current base image has been deprecated in favor of openjdk alternative.

This image is officially deprecated in favor of the openjdk image, and will receive no further updates after 2016-12-31 (Dec 31, 2016). Please adjust your usage accordingly.

Maybe 8-jdk-alpine is a good option (although it may require install additional software).

This image is based on the popular Alpine Linux project, available in the alpine official image. Alpine Linux is much smaller than most distribution base images (~5MB), and thus leads to much slimmer images in general.