docs: add basic docker-container driver guide

Signed-off-by: Justin Chadwell <me@jedevc.com>

docs/guides/drivers/docker-container.md

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+# Docker container driver
+
+The buildx docker-container driver allows creation of a managed and
+customizable BuildKit environment inside a dedicated Docker container.
+
+Using the docker-container driver has a couple of advantages over the basic
+docker driver. Firstly, we can manually override the version of buildkit to
+use, meaning that we can access the latest and greatest features as soon as
+they're released, instead of waiting to upgrade to a newer version of Docker.
+Additionally, we can access more complex features like multi-architecture
+builds and the more advanced cache exporters, which are currently unsupported
+in the default docker driver.
+
+We can easily create a new builder that uses the docker-container driver:
+
+```console
+$ docker buildx create --name container --driver docker-container
+container
+```
+
+We should then be able to see it on our list of available builders:
+
+```console
+$ docker buildx ls
+NAME/NODE       DRIVER/ENDPOINT      STATUS   BUILDKIT PLATFORMS
+container       docker-container                       
+  container0    desktop-linux        inactive          
+default         docker                                 
+  default       default              running  20.10.17 linux/amd64, linux/386
+```
+
+If we trigger a build, the appropriate `moby/buildkit` image will be pulled
+from [Docker Hub](https://hub.docker.com/u/moby/buildkit), the image started,
+and our build submitted to our containerized build server.
+
+```console
+$ docker buildx build -t <image> --builder=container .
+WARNING: No output specified with docker-container driver. Build result will only remain in the build cache. To push result image into registry use --push or to load image into docker use --load
+#1 [internal] booting buildkit
+#1 pulling image moby/buildkit:buildx-stable-1
+#1 pulling image moby/buildkit:buildx-stable-1 1.9s done
+#1 creating container buildx_buildkit_container0
+#1 creating container buildx_buildkit_container0 0.5s done
+#1 DONE 2.4s
+...
+```
+
+Note the warning "Build result will only remain in the build cache" - unlike
+the `docker` driver, the built image must be explicitly loaded into the local
+image store. We can use the `--load` flag for this:
+
+```console
+$ docker buildx build --load -t <image> --builder=container .
+...
+ => exporting to oci image format                                                                                                      7.7s
+ => => exporting layers                                                                                                                4.9s
+ => => exporting manifest sha256:4e4ca161fa338be2c303445411900ebbc5fc086153a0b846ac12996960b479d3                                      0.0s 
+ => => exporting config sha256:adf3eec768a14b6e183a1010cb96d91155a82fd722a1091440c88f3747f1f53f                                        0.0s 
+ => => sending tarball                                                                                                                 2.8s 
+ => importing to docker   
+```
+
+The image should then be available in the image store:
+
+```console
+$ docker image ls
+REPOSITORY                       TAG               IMAGE ID       CREATED             SIZE
+<image>                          latest            adf3eec768a1   2 minutes ago       197MB
+```