Docker image for linux

In Linux, compiling a binary against glibc fixes the binary to that version of glibc or any version after it (see #8). So in order to have broad binary compatibility, canvas-prebuilt has to be compiled against the oldest reasonable version of glibc.

Currently, that's done by compiling on an LTS release of Debian. Since we still want the newest versions of Pango, Cairo, etc., that means each dependency has to be downloaded and compiled manually. The Dockerfile is in the docker folder of the repo.