pkglib: untangle WithBuildDocker and DryRun

this makes it possible for a user of this API to
build their own DryRunner

Signed-off-by: Christoph Ostarek <christoph@zededa.com>

Author: christoph-zededa

src/cmd/linuxkit/pkg_build.go

@@ -115,6 +115,8 @@ func pkgBuildCmd() *cobra.Command {
 
 			if dryRun {
 				opts = append(opts, pkglib.WithDryRun())
+				opts = append(opts, pkglib.WithBuildDocker(pkglib.NewDockerDryRunner()))
+
 			}
 
 			// read any build arg files

src/cmd/linuxkit/pkglib/build.go

@@ -318,10 +318,7 @@ func (p Pkg) Build(bos ...BuildOpt) error {
 	}
 
 	d := bo.runner
-	switch {
-	case bo.dryRun:
-		d = newDockerDryRunner()
-	case d == nil:
+	if d == nil {
 		d = newDockerRunner(p.cache)
 	}
 

src/cmd/linuxkit/pkglib/dockerdryrun.go

@@ -25,7 +25,7 @@ import (
 type dockerDryRunnerImpl struct {
 }
 
-func newDockerDryRunner() DockerRunner {
+func NewDockerDryRunner() DockerRunner {
 	return &dockerDryRunnerImpl{}
 }