release: use "runtime" instead of "minimal" for OCI image name

The runtime name is taken from the main pkg-base package that this
image is built off.

Sponsored by: SkunkWerks, GmbH
MFC after: 3 days
Reviewed by: dfr, emaste
Differential Revision: https://reviews.freebsd.org/D50043

(cherry picked from commit a0165254bfeb5e310f92d4e0c88fcb5c6ea802bf)