Update alpine Docker tag to v3.15

Dockerfile

@@ -10,7 +10,7 @@ COPY . /src
 WORKDIR /src
 RUN make build
 
-FROM alpine:3.14
+FROM alpine:3.15
 RUN mkdir /lib64
 RUN ln -s /lib/libc.musl-x86_64.so.1 /lib64/ld-linux-x86-64.so.2
 COPY --from=builder /src/messagen.bin /usr/local/bin/messagen