fixup cache sharing mode

Signed-off-by: Alex Suraci <suraci.alex@gmail.com>

core/cache.go

@@ -75,6 +75,21 @@ func (mode CacheSharingMode) MarshalJSON() ([]byte, error) {
 	return json.Marshal(strings.ToLower(string(mode)))
 }
 
+// CacheSharingMode marshals to its lowercased value.
+//
+// NB: as far as I can recall this is purely for ~*aesthetic*~. GraphQL consts
+// are so shouty!
+func (mode *CacheSharingMode) UnmarshalJSON(payload []byte) error {
+	var str string
+	if err := json.Unmarshal(payload, &str); err != nil {
+		return err
+	}
+
+	*mode = CacheSharingMode(strings.ToUpper(str))
+
+	return nil
+}
+
 func (cache *CacheVolume) WithKey(key string) *CacheVolume {
 	cache = cache.Clone()
 	cache.Keys = append(cache.Keys, key)

core/container.go

@@ -572,6 +572,10 @@ func (container *Container) WithMountedCache(ctx context.Context, gw bkgw.Client
 
 	target = absPath(container.Config.WorkingDir, target)
 
+	if sharingMode == "" {
+		sharingMode = CacheSharingModeShared
+	}
+
 	mount := ContainerMount{
 		Target:           target,
 		CacheID:          cache.Sum(),