The option --add-host and --net=host should not be mutually exclusive.

This fix tries to address the issue raised in #21976 and allows
the options of `--add-host` and `--net=host` to work at the same time.

The documentation has been updated and additional tests have been
added to cover this change.

This fix fixes #21976.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

docs/reference/run.md

@@ -382,14 +382,14 @@ name, they must be linked.
 With the network set to `host` a container will share the host's
 network stack and all interfaces from the host will be available to the
 container.  The container's hostname will match the hostname on the host
-system.  Note that `--add-host` and `--mac-address` are invalid in `host` netmode. Even in `host`
+system. Note that `--mac-address` is invalid in `host` netmode. Even in `host`
 network mode a container has its own UTS namespace by default. As such
 `--hostname` is allowed in `host` network mode and will only change the
 hostname inside the container.
-Note also that `--dns`, `--dns-search` and `--dns-opt` are
-valid in `host` mode and `/etc/resolv.conf` will be updated accordingly. However, the
-update in `/etc/resolv.conf` only happens inside the container. No change will be
-made for `/etc/resolv.conf` in host.
+Similar to `--hostname`, the `--add-host`, `--dns`, `--dns-search`, and
+`--dns-opt` options can be used in `host` network mode. These options update
+`/etc/hosts` or `/etc/resolv.conf` inside the container. No change are made to
+`/etc/hosts` and `/etc/resolv.conf` on the host.
 
 Compared to the default `bridge` mode, the `host` mode gives *significantly*
 better networking performance since it uses the host's native networking stack