Document `SYNAPSE_LOG_LEVEL` to configure logging severity in Docker

[MadLittleMods]

Description:

SYNAPSE_LOG_LEVEL is used in docker/conf/log.config to set the logger severity level but is not documented anywhere.

The log severity can also be overriden by setting a log_config in homeserver.yaml (SYNAPSE_CONFIG_PATH)

SYNAPSE_CONFIG_PATH: path to the file to be generated. Defaults to <SYNAPSE_CONFIG_DIR>/homeserver.yaml.

Dev notes

• SYNAPSE_LOG_LEVEL was first introduceded in 4f2e898
• Changed a bit in https://github.com/matrix-org/synapse/pull/5547/files
• Someone noted as broken in Logging broken in Docker image #6194 and maybe fixed in Fix logging config for the docker image #6197

Alternative

Remove this config altogether in favor of having to specify a log_config in homeserver.yaml

Definition of done

We have a nice list of variables on https://hub.docker.com/r/matrixdotorg/synapse which this would fit nicely in.

• Document SYNAPSE_LOG_LEVEL in docker/README.md

[MadLittleMods]

Related MR that documented SYNAPSE_TEST_LOG_LEVEL, #10148

[MadLittleMods]

This was documented via #13152 (https://github.com/matrix-org/synapse/pull/13152/files#diff-da8fcbe728a9172b578e5d754f8e2df214c658c4321f610e63dd68bea828ab49R70)