Bump stalwartlabs/mail-server from v0.9.0 to v0.9.1 (#16)

Bumps stalwartlabs/mail-server from v0.9.0 to v0.9.1.

---
updated-dependencies:
- dependency-name: stalwartlabs/mail-server
  dependency-type: direct:production
...

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

Dockerfile

@@ -1,5 +1,5 @@
 # From https://github.com/stalwartlabs/mail-server/blob/main/Dockerfile
-FROM stalwartlabs/mail-server:v0.9.0
+FROM stalwartlabs/mail-server:v0.9.1
 
 COPY --chmod=664 config.toml /opt/stalwart-mail/etc/config.toml
 COPY --chmod=775 start.sh /start.sh