Add ATTACHMENTS_STORE_PATH to Dockerfile.

Dockerfile

@@ -22,6 +22,7 @@ ENV BUILD_DEPS="apt-utils bsdtar gnupg gosu wget curl bzip2 g++ build-essential
     ACCOUNTS_LOCKOUT_UNKNOWN_USERS_LOCKOUT_PERIOD=60 \
     ACCOUNTS_LOCKOUT_UNKNOWN_USERS_FAILURE_WINDOW=15 \
     RICHER_CARD_COMMENT_EDITOR=true \
+    ATTACHMENTS_STORE_PATH="" \
     MAX_IMAGE_PIXEL="" \
     IMAGE_COMPRESS_RATIO="" \
     BIGEVENTS_PATTERN="" \