docs: add documentation for --buildkitd-flags, --config, --driver-opt on create

Signed-off-by: Tibor Vass <tibor@docker.com>

README.md

@@ -332,13 +332,41 @@ Options:
 | Flag | Description |
 | --- | --- |
 | --append                 | Append a node to builder instead of changing it
+| --buildkitd-flags string | Flags for buildkitd daemon
+| --config string          | BuildKit config file
 | --driver string          | Driver to use (eg. docker-container)
+| --driver-opt stringArray | Options for the driver
 | --leave                  | Remove a node from builder instead of changing it
 | --name string            | Builder instance name
 | --node string            | Create/modify node with given name
 | --platform stringArray   | Fixed platforms for current node
 | --use                    | Set the current builder instance
 
+#### `--append`
+
+Changes the action of the command to appends a new node to an existing builder specified by `--name`. Buildx will choose an appropriate node for a build based on the platforms it supports.
+
+Example:
+```
+$ docker buildx create mycontext1
+eager_beaver
+$ docker buildx create --name eager_beaver --append mycontext2
+eager_beaver
+```
+
+#### `--buildkitd-flags FLAGS`
+
+Adds flags when starting the buildkitd daemon. They take precedence over the configuration file specified by `--config`. See `buildkitd --help` for the available flags.
+
+Example:
+```
+--buildkitd-flags '--debug --debugaddr 0.0.0.0:6666'
+```
+
+#### `--config FILE`
+
+Specifies the configuration file for the buildkitd daemon to use. The configuration can be overridden by `--buildkitd-flags`. See an [example buildkitd configuration file](https://github.com/moby/buildkit/blob/master/docs/buildkitd.toml.md).
+
 #### `--driver DRIVER`
 
 Sets the builder driver to be used. There are two available drivers, each have their own specificities.
@@ -347,16 +375,18 @@ Sets the builder driver to be used. There are two available drivers, each have t
 
 - `docker-container` - Uses a buildkit container that will be spawned via docker. With this driver, both building multi-platform images and exporting cache are supported. However, images built will not automatically appear in `docker images` (see [`build --load`](#--load)).
 
-#### `--append`
 
-Changes the action of the command to appends a new node to an existing builder specified by `--name`. Buildx will choose an appropriate node for a build based on the platforms it supports.
+#### `--driver-opt OPTIONS`
 
-Example:
+Passes additional driver-specific options. Details for each driver:
+
+- `docker` - No driver options
+- `docker-container`
+  - `image` - Sets the container image to be used for running buildkit.
+  - `network` - Sets the network mode for running the buildkit container.
+  - Example:
     ```
-$ docker buildx create mycontext1
-eager_beaver
-$ docker buildx create --name eager_beaver --append mycontext2
-eager_beaver
+    --driver docker-container --driver-opt image=moby/buildkit:master,network=host
     ```
 
 #### `--leave`