fix: check bind source file/dir exists

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/stacker/build.go

@@ -820,6 +820,12 @@ func SetupLayerConfig(config types.StackerConfig, c *container.Container, l type
 	}
 
 	for _, bind := range l.Binds {
+		_, err := os.Stat(bind.Source)
+		// c.BindMount allows files to not exist and will create them, but not so for directories.
+		if os.IsNotExist(err) {
+			log.Errorf("bind mount source %q does not exist, refusing bind mount: %s", bind.Source, err)
+			return fmt.Errorf("bind mount source %q does not exist, refusing bind mount: %s", bind.Source, err)
+		}
 		err = c.BindMount(bind.Source, bind.Dest, "")
 		if err != nil {
 			return err