fix batching; work around many other buildx glitches; pin demo os version

Author: Andrew Pennebaker

example/Dockerfile

@@ -1,2 +1,2 @@
-FROM busybox
+FROM busybox:1.34
 RUN echo "Hello World!" >/banner

job.go

@@ -79,7 +79,7 @@ func (o Job) runBatch() error {
 		return cmd.Run()
 	}
 
-	cmd.Args = append(cmd.Args, "build")
+	cmd.Args = append(cmd.Args, "build", "--builder", TugBuilderName)
 
 	var platformPairs []string
 
@@ -165,6 +165,12 @@ func (o Job) Run() error {
 		o.Platforms = o.Platforms[batchSize:]
 	}
 
+	//
+	// Work around corruption glitch in buildx --push.
+	//
+	push := o.Push
+	o.Push = false
+
 	for _, platformGroup := range platformGroups {
 		o.Platforms = platformGroup
 
@@ -173,5 +179,11 @@ func (o Job) Run() error {
 		}
 	}
 
+	if push {
+		o.Push = true
+		o.Platforms = platforms
+		return o.runBatch()
+	}
+
 	return nil
 }

platform.go

@@ -70,8 +70,8 @@ func (o Platforms) Less(i int, j int) bool {
 	return o[i].Format() < o[j].Format()
 }
 
-// EnsureTugBuilderExists creates a tug buildx builder when necessary.
-func EnsureTugBuilderExists() error {
+// EnsureTugBuilder prepares the tug buildx builder.
+func EnsureTugBuilder() error {
 	cmd := exec.Command("docker")
 	cmd.Args = []string{"docker", "buildx", "ls"}
 	cmd.Env = os.Environ()
@@ -105,7 +105,7 @@ func EnsureTugBuilderExists() error {
 
 	if !foundTugBuilder {
 		cmd := exec.Command("docker")
-		cmd.Args = []string{"docker", "buildx", "create", "--name", TugBuilderName}
+		cmd.Args = []string{"docker", "buildx", "create", "--bootstrap", "--name", TugBuilderName}
 		cmd.Env = os.Environ()
 		cmd.Stderr = os.Stderr
 		return cmd.Run()
@@ -114,37 +114,6 @@ func EnsureTugBuilderExists() error {
 	return nil
 }
 
-// EnsureTugBuilderInUse activates the tug buildx builder.
-func EnsureTugBuilderInUse() error {
-	cmd := exec.Command("docker")
-	cmd.Args = []string{"docker", "buildx", "use", TugBuilderName}
-	cmd.Env = os.Environ()
-	cmd.Stderr = os.Stderr
-	return cmd.Run()
-}
-
-// EnsureTugBuilderBootstrapped prepares the tug buildx builder for building.
-func EnsureTugBuilderBootstrapped() error {
-	cmd := exec.Command("docker")
-	cmd.Args = []string{"docker", "buildx", "inspect", "--bootstrap"}
-	cmd.Env = os.Environ()
-	cmd.Stderr = os.Stderr
-	return cmd.Run()
-}
-
-// EnsureTugBuilder fully initializes the tug buildx builder.
-func EnsureTugBuilder() error {
-	if err := EnsureTugBuilderExists(); err != nil {
-		return err
-	}
-
-	if err := EnsureTugBuilderInUse(); err != nil {
-		return err
-	}
-
-	return EnsureTugBuilderBootstrapped()
-}
-
 // AvailablePlatforms initializes tug and reports the available buildx platforms.
 func AvailablePlatforms() ([]Platform, error) {
 	if err := EnsureTugBuilder(); err != nil {
@@ -200,6 +169,5 @@ func AvailablePlatforms() ([]Platform, error) {
 	}
 
 	sort.Sort(Platforms(platforms))
-
 	return platforms, nil
 }