Is it possible to increase Redis `maxmemory` configuration option?

I tried bumping up cluster size to L and adding a configuration: { maxmemory: 134217728 } to redis service but I did not manage to change the default 28MB limit.

Unfortunately the answer is no, the maxmemory for Redis isn’t configurable. However, you could try increasing the container size, which should increase the maxmemory for the service. The Redis service will use all the available memory in the container, so container sizing is the way to increase that.