.devcontainer, update image to ubuntu:rolling

- see also: 8446640060e6e5058d0d186d71831a32a9e606e3

.devcontainer/Dockerfile

@@ -1,4 +1,4 @@
-FROM quay.io/wekan/ubuntu:groovy-20210115
+FROM ubuntu:rolling
 LABEL maintainer="sgr"
 
 ENV BUILD_DEPS="gnupg gosu libarchive-tools wget curl bzip2 g++ build-essential python git ca-certificates iproute2"