Update Ubuntu image to 20.04

Author: JohnTitor

ci/docker/i686-unknown-linux-gnu/Dockerfile

@@ -1,6 +1,4 @@
-# FIXME: Somehow we encounter a panic with Ubuntu 20.04.
-# Should investigate why it causes and fix.
-FROM ubuntu:19.10
+FROM ubuntu:20.04
 RUN apt-get update
 RUN apt-get install -y --no-install-recommends \
   gcc-multilib libc6-dev ca-certificates