build: Fix rpath to refer to absolute location

We originally set the rpath to be relative to the $ORIGIN which
apparently changes if the working directory is overriden. This change
just fixes the rpath to contain /usr/lib/opa and then updates the
Dockerfile to copy the libwasmer.so file into this new directory.

Fixes #2974

Signed-off-by: Torin Sandall <torinsandall@gmail.com>