Five new docker-related slave flags are not covered by the configuration documentation.

These flags were added to "slave/flags.cpp", but are not mentioned in "docs/configuration.md":

add(&Flags::docker_auth_server,
"docker_auth_server",
"Docker authentication server",
"auth.docker.io");

add(&Flags::docker_auth_server_port,
"docker_auth_server_port",
"Docker authentication server port",
"443");
add(&Flags::docker_puller_timeout_secs,
"docker_puller_timeout",
"Timeout value in seconds for pulling images from Docker registry",
"60");

add(&Flags::docker_registry,
"docker_registry",
"Default Docker image registry server host",
"registry-1.docker.io");
add(&Flags::docker_registry_port,
"docker_registry_port",
"Default Docker registry server port",
"443");