To delete all keys in your redis db, just enter FLUSHDB. This was useful when I want to start anew with my local testing.
docker exec -it redis redis-cli
127.0.0.1:6379> SET key1 value1
OK
127.0.0.1:6379> SET key2 value2
OK
127.0.0.1:6379> GET key1
"value1"
127.0.0.1:6379> KEYS *
1) "key1"
2) "key2"
127.0.0.1:6379> FLUSHDB
OK
127.0.0.1:6379> KEYS *
(empty array)
127.0.0.1:6379>