As per the first paragraph in the linked documentation:
You can either log to a syslog server via UDP (IP:port) or UNIX socket, with UDP being recommended as UNIX sockets have all kinds of problems (mainly syslogd restart while haproxy is chrooted and can’t reconnect afterwards and slow syslog servers discarding log messages).
/dev/log is not a file, but a unix socket.
Logging to stdout is not supported.
Appended keywords like local0 and notice are either syslog facilities or security levels:
To log from a docker container, you should use an external syslog. If you need it within the container, check the workarounds proposed here: