Makefile... support custom prefix

[rjsmith1999]

the cmake interface supports

• cmake --install build --prefix <prefix>
• cmake --install build # default prefix of /opt/resolve/

The Makefile only supports the default /opt/resolve prefix, but we should also support:

• Custom prefix (via RESOLVE_PREFIX env var or something)
• Local prefix (./install) Resolved by add install-local target #169

[rjsmith1999]

As part of this, we should make RESOLVE_PREFIX customizable in the dockerfile...