docs: user guides

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

docs/guides/custom-registry-config.md

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+# Using a custom registry configuration
+
+If you [create a `docker-container` or `kubernetes` builder](../reference/buildx_create.md) and
+have specified certificates for registries in the [BuildKit daemon configuration](https://github.com/moby/buildkit/blob/master/docs/buildkitd.toml.md),
+the files will be copied into the container under `/etc/buildkit/certs` and
+configuration will be updated to reflect that.
+
+Take the following `buildkitd.toml` configuration that will be used for
+pushing an image to this registry using self-signed certificates:
+
+```toml"
+debug = true
+[registry."myregistry.com"]
+  ca=["/etc/certs/myregistry.pem"]
+  [[registry."myregistry.com".keypair]]
+    key="/etc/certs/myregistry_key.pem"
+    cert="/etc/certs/myregistry_cert.pem"
+```
+> `/etc/buildkitd.toml`
+
+Here we have configured a self-signed certificate for `myregistry.com` registry.
+
+Now [create a `docker-container` builder](../reference/buildx_create.md)
+that will use this BuildKit configuration:
+
+```console
+$ docker buildx create --use \
+  --name mybuilder \
+  --driver docker-container \
+  --config /etc/buildkitd.toml
+```
+
+Inspecting the builder container, you can see that buildkitd configuration
+has changed:
+
+```console
+$ docker exec -it buildx_buildkit_mybuilder0 cat /etc/buildkit/buildkitd.toml
+```
+```toml
+debug = true
+
+[registry]
+
+  [registry."myregistry.com"]
+    ca = ["/etc/buildkit/certs/myregistry.com/myregistry.pem"]
+
+    [[registry."myregistry.com".keypair]]
+      cert = "/etc/buildkit/certs/myregistry.com/myregistry_cert.pem"
+      key = "/etc/buildkit/certs/myregistry.com/myregistry_key.pem"
+```
+
+And certificates copied inside the container:
+
+```console
+$ docker exec -it buildx_buildkit_mybuilder0 ls /etc/buildkit/certs/myregistry.com/
+myregistry.pem    myregistry_cert.pem   myregistry_key.pem
+```
+
+Now you should be able to push to the registry with this builder:
+
+```console
+$ docker buildx build --push --tag myregistry.com/myimage:latest .
+```