Bump alpine from 3.15.1 to 3.15.2

Bumps alpine from 3.15.1 to 3.15.2.

---
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 alpine:3.15.1 as build
+FROM alpine:3.15.2 as build
 
 RUN apk add --no-cache \
     cmake \
@@ -18,7 +18,7 @@ RUN mkdir build \
     && make
 
 
-FROM alpine:3.15.1 as service
+FROM alpine:3.15.2 as service
 
 RUN adduser -S milterfrom \
     && apk add libmilter