Fix environment error for alpine and armhf

Dockerfile.alpine

@@ -103,7 +103,8 @@ ENV OPENVPN_USERNAME=**None** \
     TRANSMISSION_WEB_UI= \
     PUID= \
     PGID= \
-    TRANSMISSION_WEB_HOME=
+    TRANSMISSION_WEB_HOME= \
+    DROP_DEFAULT_ROUTE=
 
 # Expose port and run
 EXPOSE 9091

Dockerfile.armhf

@@ -105,7 +105,8 @@ ENV OPENVPN_USERNAME=**None** \
     TRANSMISSION_WEB_UI=\
     PUID=\
     PGID=\
-    TRANSMISSION_WEB_HOME=
+    TRANSMISSION_WEB_HOME= \
+    DROP_DEFAULT_ROUTE=
 
 # Expose port and run
 EXPOSE 9091