driver: docker-container driver uses --config correctly in rootless mode

The `docker-container` driver relies on the default config file location
for buildkit when writing the configuration file. When run in a rootless
version of docker (dind), the default location is different.

Instead of trying to figure out where the appropriate default location
is, this just writes the files to the same location and sets the
`--config` parameter explicitly. This flag is placed first so a
user-specified config option in `--buildkitd-flags` will take precedence
over the implicit config parameter.

Signed-off-by: Jonathan A. Sternberg <[email protected]>

driver/docker-container/driver.go

@@ -33,7 +33,8 @@ import (
 )
 
 const (
-	volumeStateSuffix = "_state"
+	volumeStateSuffix   = "_state"
+	buildkitdConfigFile = "buildkitd.toml"
 )
 
 type Driver struct {
@@ -114,9 +115,7 @@ func (d *Driver) create(ctx context.Context, l progress.SubLogger) error {
 		Image: imageName,
 		Env:   d.env,
 	}
-	if d.InitConfig.BuildkitFlags != nil {
-		cfg.Cmd = d.InitConfig.BuildkitFlags
-	}
+	cfg.Cmd = getBuildkitFlags(d.InitConfig)
 
 	useInit := true // let it cleanup exited processes created by BuildKit's container API
 	if err := l.Wrap("creating container "+d.Name, func() error {
@@ -487,3 +486,14 @@ func writeConfigFiles(m map[string][]byte) (_ string, err error) {
 	}
 	return tmpDir, nil
 }
+
+func getBuildkitFlags(initConfig driver.InitConfig) []string {
+	flags := initConfig.BuildkitFlags
+	if _, ok := initConfig.Files[buildkitdConfigFile]; ok {
+		// Be explicit with the config file if it exists.
+		newFlags := make([]string, 0, len(flags)+2)
+		newFlags = append(newFlags, "--config", path.Join("/etc/buildkit", buildkitdConfigFile))
+		flags = append(newFlags, flags...)
+	}
+	return flags
+}