dockerfile: use Alpine 3.18 as base for release image (#661)

Dockerfile

@@ -4,7 +4,7 @@ RUN apk add --no-cache git make && \
     cd /s5cmd/ && \
     CGO_ENABLED=0 make build
 
-FROM alpine:3.15
+FROM alpine:3.18
 COPY --from=build /s5cmd/s5cmd .
 WORKDIR /aws
 ENTRYPOINT ["/s5cmd"]