Bump ubuntu from b795f8e to db8bf6f in /dev/ci/docker_linux (#128740)

Bumps ubuntu from `b795f8e` to `db8bf6f`.

[![Dependabot compatibility score](https://dependabot-badges.githubapp.com/badges/compatibility_score?dependency-name=ubuntu&package-manager=docker&previous-version=focal&new-version=focal)](https://docs.github.com/en/github/managing-security-vulnerabilities/about-dependabot-security-updates#about-compatibility-scores)

Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting `@dependabot rebase`.

---

<details>
<summary>Dependabot commands and options</summary>
<br />

You can trigger Dependabot actions by commenting on this PR:
- `@dependabot rebase` will rebase this PR
- `@dependabot recreate` will recreate this PR, overwriting any edits that have been made to it
- `@dependabot merge` will merge this PR after your CI passes on it
- `@dependabot squash and merge` will squash and merge this PR after your CI passes on it
- `@dependabot cancel merge` will cancel a previously requested merge and block automerging
- `@dependabot reopen` will reopen this PR if it is closed
- `@dependabot close` will close this PR and stop Dependabot recreating it. You can achieve the same result by closing it manually
- `@dependabot ignore this major version` will close this PR and stop Dependabot creating any more for this major version (unless you reopen the PR or upgrade to it yourself)
- `@dependabot ignore this minor version` will close this PR and stop Dependabot creating any more for this minor version (unless you reopen the PR or upgrade to it yourself)
- `@dependabot ignore this dependency` will close this PR and stop Dependabot creating any more for this dependency (unless you reopen the PR or upgrade to it yourself)

</details>

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