Allow adding arbitrary buildkit config

README.md

@@ -157,6 +157,9 @@ _(As a convention in the list below, all task parameters are specified with a
   (`,`) list of key-value pairs (using syntax `hostname=ip-address`), each
   defining an IP address for resolving some custom hostname.
 
+* `$BUILDKIT_EXTRA_CONFIG` (default empty): a string written verbatim to builkit's
+  TOML config file. See [buildkitd.toml](https://docs.docker.com/build/buildkit/toml-configuration/).
+
 > Note: this is the main pain point with reusable tasks - env vars are kind of
 > an awkward way to configure a task. Once the RFC lands these will turn into a
 > JSON structure similar to configuring `params` on a resource, and task params

buildkitd.go

@@ -173,6 +173,13 @@ func generateConfig(req Request, configPath string) error {
 		return err
 	}
 
+	if len(req.Config.BuildkitExtraConfig) > 0 {
+		_, err = f.WriteString(req.Config.BuildkitExtraConfig)
+		if err != nil {
+			return err
+		}
+	}
+
 	return f.Close()
 }
 

types.go

@@ -64,6 +64,8 @@ type Config struct {
 
 	BuildkitSecrets map[string]string `json:"buildkit_secrets" envconfig:"optional"`
 
+	BuildkitExtraConfig string `json:"buildkit_extra_config" envconfig:"BUILDKIT_EXTRA_CONFIG,optional"`
+
 	// Unpack the OCI image into Concourse's rootfs/ + metadata.json image scheme.
 	//
 	// Theoretically this would go away if/when we standardize on OCI.