Add option to build/bake to override instance

This helps prevent race conditions with concurrent build invocations.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

commands/builder.go

@@ -0,0 +1,11 @@
+package commands
+
+import "github.com/spf13/pflag"
+
+type builderOptions struct {
+	builder string
+}
+
+func builderFlags(options *builderOptions, flags *pflag.FlagSet) {
+	flags.StringVar(&options.builder, "builder", "", "Override the configured builder instance")
+}