Run `make uninstall` for oniguruma before running the recursive
distclean that will remove oniguruma's Makefile and cause a build error
due to missing make target.
Fetch oniguruma using git submodule instead of a release tarball. It
will fix a build problem, caused by jq's autotools configuration, trying
to run `make distclean` recursively in an empty modules directory. This
will also improve the maintainability of the Dockerfile.