You can configure memcached for your peer server the same way you would for any other peer application, adding the memcached (or valcache) arguments to the command line invocation, e.g.:
bin/run -Ddatomic.valcachePath=/tmp/valcache/ -Ddatomic.valcacheMaxGb=40 -Xmx8g -m datomic.peer-server -a key,secret -d mydb,datomic:dev://localhost:4334/my-db