fix: check bind source file/dir exists (project-stacker#624)

Stacker does not check if a bind mount source exists and if the source
is a directory then the container crashes with a stack trace that does
not indicate that the missing source dir is the issue.

Check if the source exists and return an error indicating the source
is missing instead of the container stack trace.

Signed-off-by: Ryan Harper <ryaharpe@cisco.com>

pkg/container/container.go

@@ -65,6 +65,9 @@ func New(sc types.StackerConfig, name string) (*Container, error) {
 func (c *Container) BindMount(source string, dest string, extraOpts string) error {
 	createOpt := "create=dir"
 	stat, err := os.Stat(source)
+	if os.IsNotExist(err) {
+		return errors.Errorf("bind mount source %q does not exist, refusing bind mount: %s", source, err)
+	}
 	if err == nil && !stat.IsDir() {
 		createOpt = "create=file"
 	}

pkg/stacker/build.go

@@ -820,9 +820,10 @@ func SetupLayerConfig(config types.StackerConfig, c *container.Container, l type
 	}
 
 	for _, bind := range l.Binds {
+		log.Debugf("bind mounting %q into container at %q", bind.Source, bind.Dest)
 		err = c.BindMount(bind.Source, bind.Dest, "")
 		if err != nil {
-			return err
+			return errors.Errorf("failed to bind mount %q at %q: %s", bind.Source, bind.Dest, err)
 		}
 	}