Bump trillian-opensource-ci/mysql5 (#3171)

Bumps trillian-opensource-ci/mysql5 from `c079e4e` to `3f355be`.

---
updated-dependencies:
- dependency-name: trillian-opensource-ci/mysql5
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

examples/deployment/docker/db_server/Dockerfile

@@ -1,4 +1,4 @@
-FROM gcr.io/trillian-opensource-ci/mysql5:5.7@sha256:c079e4e1cfa0e244f859fe37dc28d9c133835bc34098196132c461db91dd85f3
+FROM gcr.io/trillian-opensource-ci/mysql5:5.7@sha256:3f355bed2d85d4ad266771b3c7ddf4251ee5d8a45afc140336d1c0cab9dc5171
 
 # Patch the OS-level packages and remove unneeded dependencies.
 ENV DEBIAN_FRONTEND=noninteractive