Refine docker detect mechenism. v5.0.190 v6.0.90 (#3758)

When using Docker, logs are usually printed to console (stdout and
stderr). However, since Docker detection occurs late, after log
initialization, the default log output may be incorrect. In Docker, logs
may still be written to a file instead of the console as expected.

Additionally, the Dockerfile has been improved with a new environment
variable `SRS_IN_DOCKER=on` to clearly indicate a Docker environment. If
automatic Docker detection fails, the configuration will be read, and
this variable will correctly inform SRS that it's in a Docker
environment.

Lastly, the default configuration values have been improved for Docker
environments. By default, `SRS_LOG_TANK=console` and daemon mode is
disabled.

---------

Co-authored-by: john <hondaxiao@tencent.com>

Dockerfile

@@ -56,6 +56,6 @@ RUN ldd /usr/local/srs/objs/ffmpeg/bin/ffmpeg && \
 
 # Default workdir and command.
 WORKDIR /usr/local/srs
-ENV SRS_DAEMON=off
+ENV SRS_DAEMON=off SRS_IN_DOCKER=on
 CMD ["./objs/srs", "-c", "conf/docker.conf"]