fix the build and improve the build process

This PR fixes the broken build caused by the bump to live-build 4 alpha in jessie. This was fixed by making the Dockerfile use the wheezy image to get live-build 3 and use it to prepare the jessie based iso image.

This PR also makes a few other changes:
- stop building the zsync file; this speeds up the build a bit
- switch the mirror to http.debian.net to make downloads faster; this speeds up the build
- replace acpi support hacks with acpi-support-base to handle clean shutdown
- rename the output image to `debian2docker.iso` and remove the rename step from the readme
- clean up some more unneeded files during build