Ease usage with docker entrypoint and env vars

[barcus]

Hello @vierbergenlars, thank you for this nice exporter :)
This MR to ease usage of the docker image using environment variables.
CMD can be overridden using dedicated command in docker run or docker-compose file.
I fact would like to add this exporter in my docker-compose file (https://github.com/barcus/bareos)

• add entrypoint script to manage exporter args
• freeeze golang and busybox images version
• add WAIT_FOR_DB variable
• use upx to compress exporter executable

[vierbergenlars]

Hi, thanks for your PR to make the docker image more usable.

I have one blocking remark: the backwards compatibility of the Docker image must be retained, as it is used in production.

My other documentation remarks are more suggestions on how to improve the docs.

[vierbergenlars]

Thanks, looks good now!