Bump alpine from 3.15.0 to 3.15.1

Bumps alpine from 3.15.0 to 3.15.1.

---
updated-dependencies:
- dependency-name: alpine
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Dockerfile

@@ -1,4 +1,4 @@
-FROM docker.io/alpine:3.15.0
+FROM docker.io/alpine:3.15.1
 
 ARG DOVECOT_PACKAGE_VERSION=2.3.17.1-r0
 ARG SOCAT_PACKAGE_VERSION=1.7.4.2-r0