Reduce APIcast image size

The current production image is current around ~750MB

registry.redhat.io/3scale-amp2/apicast-gateway-rhel8   1.24.0-19             d821225d00c7   3 months ago     775MB

And a big part of it is because we copy the entire cachito cache into the final image, which then creates a fat docker layer due to the COPY command.

COPY $REMOTE_SOURCES $REMOTE_SOURCES_DIR

If we only copy APIcast source from cachito we can shave off ~350MB from the final image.

COPY $REMOTE_SOURCES/apicast $REMOTE_SOURCES_DIR/apicast