Make sure we error out instead of panic during interpolation

Use type assertion to error out if the type isn't the right one
instead of panic as before this change.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

compose/interpolation/interpolation.go

@@ -1,9 +1,8 @@
 package interpolation
 
 import (
-	"fmt"
-
 	"github.com/docker/docker/cli/compose/template"
+	"github.com/pkg/errors"
 )
 
 // Interpolate replaces variables in a string with the values from a mapping
@@ -15,7 +14,11 @@ func Interpolate(config map[string]interface{}, section string, mapping template
 			out[name] = nil
 			continue
 		}
-		interpolatedItem, err := interpolateSectionItem(name, item.(map[string]interface{}), section, mapping)
+		mapItem, ok := item.(map[string]interface{})
+		if !ok {
+			return nil, errors.Errorf("Invalid type for %s : %T instead of %T", name, item, out)
+		}
+		interpolatedItem, err := interpolateSectionItem(name, mapItem, section, mapping)
 		if err != nil {
 			return nil, err
 		}
@@ -37,7 +40,7 @@ func interpolateSectionItem(
 	for key, value := range item {
 		interpolatedValue, err := recursiveInterpolate(value, mapping)
 		if err != nil {
-			return nil, fmt.Errorf(
+			return nil, errors.Errorf(
 				"Invalid interpolation format for %#v option in %s %#v: %#v. You may need to escape any $ with another $.",
 				key, section, name, err.Template,
 			)