* Allow passing SYNAPSE_WORKER envvar
* changelog.d
* Document SYNAPSE_WORKER.
Attempting to imply that you don't need to change this default
unless you're in worker mode.
Also aware that there's a bigger problem of attempting to document
a complete working configuration of workers using docker, as we
currently only document to use `synctl` for worker mode, and synctl
doesn't work that way in docker.
This is mostly a documentation change, but also adds a default value for
SYNAPSE_CONFIG_PATH, so that running from the generated config is the default,
and will Just Work provided your config is in the right place.
When running under docker, we want to use docker's own logging stuff rather
than losing the logs somewhere on the container's filesystem, so let's use log
configs that spit logs out to stdout instead.
We don't want to generate any missing configs when running from a precanned
config.
(There's a strong argument that we don't want to do this at all, since
generating a new signing key on each invocation sounds disasterous, but I don't
fancy unpicking that for now.)
