docs: document more buildx targets
**- What I did**
Document more of the `buildx bake` targets introduced in the repo.

**- A picture of a cute animal (not mandatory but encouraged)**
![](https://media.discordapp.net/attachments/237010660347609088/1121827674579554325/20230622_104621.jpg?width=555&height=740)
docs/contributing/ctn-build.md
The `Dockerfile` supports building and cross compiling docker daemon and extra
tools using [Docker Buildx](https://github.com/docker/buildx) and [BuildKit](https://github.com/moby/buildkit).
A [bake definition](https://docs.docker.com/build/bake/file-definition/) named
`docker-bake.hcl` is in place to ease the build process:

```shell
# build binaries for the current host platform
# output to ./bundles/binary-daemon by default
docker buildx bake
# or
docker buildx bake binary

# build binaries for the current host platform
# output to ./bin
DESTDIR=./bin docker buildx bake

# build dynamically linked binaries
# output to ./bundles/dynbinary-daemon by default
DOCKER_STATIC=0 docker buildx bake
# or
docker buildx bake dynbinary

# build binaries for all supported platforms
docker buildx bake binary-cross

# build binaries for a specific platform
docker buildx bake --set *.platform=linux/arm64
```

The `Dockerfile` supports building and cross compiling docker daemon and extra
tools using [Docker Buildx](https://github.com/docker/buildx) and [BuildKit](https://github.com/moby/buildkit).
A [bake definition](https://docs.docker.com/build/bake/file-definition/) named
`docker-bake.hcl` is in place to ease the build process:

```shell
# build binaries for the current host platform
# output to ./bundles/binary-daemon by default
docker buildx bake
# or
docker buildx bake binary

# build binaries for the current host platform
# output to ./bin
DESTDIR=./bin docker buildx bake

# build dynamically linked binaries
# output to ./bundles/dynbinary-daemon by default
DOCKER_STATIC=0 docker buildx bake
# or
docker buildx bake dynbinary

# build binaries for all supported platforms
docker buildx bake binary-cross

# build binaries for a specific platform
docker buildx bake --set *.platform=linux/arm64

# build "complete" binaries (including containerd, runc, vpnkit, etc.)
docker buildx bake all

# build "complete" binaries for all supported platforms
docker buildx bake all-cross

# build non-runnable image wrapping "complete" binaries
# useful for use with undock and sharing via a registry
docker buildx bake bin-image

# build non-runnable image wrapping "complete" binaries, with custom tag
docker buildx bake bin-image --set "*.tags=foo/moby-bin:latest"

# build non-runnable image wrapping "complete" binaries for all supported platforms
# multi-platform images must be directly pushed to a registry
docker buildx bake bin-image-cross --set "*.tags=foo/moby-bin:latest" --push
```

Maybe specify a dummy tag as an example?

```suggestion
# build non-runnable image wrapping "complete" binaries for all supported platforms
# and push to foo/moby-bin with latest tag (multi-platform images must be directly
# pushed to a registry)
docker buildx bake bin-image-cross --set "*.tags=foo/moby-bin:latest" --push
```