support different platforms (e.g. ARM): rebased to FROM java ...

[umiddelb]

Hi,

I've filed this PR to discuss some changes which makes it easier to port docker-openhab to other platforms (e.g. ARMv7).

FROM java:7 | FROM armv7/armhf-java7 will eliminate the link to a platform dependant Java install binary in your Dockerfile. Instead of this you can play around with different Java flavours (and platforms) just by changing the FROM line and nothing else (armv7/armhf-java7 relates to my base Ubuntu Core & Oracle JDK 7 image for ARMv7 devices, e.g. the Raspberry Pi 2, or the ODROID boards).

In the consequence I needed to remove the fixed prefix to the java binary used in open openhab.conf.

Now to the downside ... the Docker Java library images are OpenJDK only, if you need Oracle JDK support, you need to build the Docker images by yourself.