How to fix «The TCP backlog setting of 511 cannot be enforced» Redis warning?

WARNING: The TCP backlog setting of 511 cannot be enforced because /proc/sys/net/core/somaxconn is set to the lower value of 128.

A docker host machine and a container shares the same linux kernel.
As such, you should be able to resolve the above warning by tuning the host kernel parameters using syscctl.
Please see docker-library/redis#35 (comment) for suggestions.

It is wrong!
The Discourse’s Docker container does not share net.core.somaxconn with the host operating system.

How to permanently set net.core.somaxconn in Debian?

./launcher rebuild <container> --docker-args "--sysctl net.core.somaxconn=65535"

«docker run» → «Configure namespaced kernel parameters (sysctls) at runtime»