How can I make my own base image for Docker?

You can take a look at how the base images are created and go from there.

You can find them here: https://github.com/dotcloud/docker/tree/master/contrib. There is mkimage-busybox.sh, mkimage-unittest.sh, mkimage-debian.sh

Quoting Solomon Hykes:

You can easily create a new container from any tarball with "docker import". For example:

debootstrap raring ./rootfs
tar -C ./rootfs -c . | docker import - flimm/mybase