Bump ubuntu from e4771b7 to 3aa2ccb in /dev/ci/docker_linux

Bumps ubuntu from `e4771b7` to `3aa2ccb`.

---
updated-dependencies:
- dependency-name: ubuntu
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

dev/ci/docker_linux/Dockerfile

@@ -12,7 +12,7 @@
 
 # Last manual update 2021-09-24 (changing this comment will re-build image)
 
-FROM ubuntu:bionic@sha256:eb1392bbdde63147bc2b4ff1a4053dcfe6d15e4dfd3cce29e9b9f52a4f88bc74
+FROM ubuntu:bionic@sha256:40b84b75884ff39e4cac4bf62cb9678227b1fbf9dbe3f67ef2a6b073aa4bb529
 MAINTAINER Flutter Developers <flutter-dev@googlegroups.com>
 
 RUN apt-get update -y && \