Bump ubuntu from b795f8e to db8bf6f in /dev/ci/docker_linux

Bumps ubuntu from `b795f8e` to `db8bf6f`.

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

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

Author: dependabot[bot]

dev/ci/docker_linux/Dockerfile

@@ -12,7 +12,7 @@
 
 # Last manual update 2021-09-24 (changing this comment will re-build image)
 
-FROM ubuntu:focal@sha256:b795f8e0caaaacad9859a9a38fe1c78154f8301fdaf0872eaf1520d66d9c0b98
+FROM ubuntu:focal@sha256:f8f658407c35733471596f25fdb4ed748b80e545ab57e84efbdb1dbbb01bd70e
 MAINTAINER Flutter Developers <flutter-dev@googlegroups.com>
 
 ENV TZ=America/Los_Angeles