Fixes for Docker build

I ran into trouble with the Dockerfile due to what seems to be a minor Debian bug in the openjdk packages.

There is a very easy workaround (one "mkdir -p") and I've included that as its own commit in the stack. I also included some tidying up that I thought to do while I was in the area. If any of that is a problem I'm happy to split this into smaller merge requests.

With this change, I can successfully build the container image locally, and then use that image to build a working APK of the onionmasq example app. APK tested in the Android emulator, using Wireshark to verify the VPN is working as expected.