fix: Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-ALPINE318-OPENSSL-5776808
- https://snyk.io/vuln/SNYK-ALPINE318-OPENSSL-5776808
- https://snyk.io/vuln/SNYK-ALPINE318-OPENSSL-5788370
- https://snyk.io/vuln/SNYK-ALPINE318-OPENSSL-5788370
- https://snyk.io/vuln/SNYK-ALPINE318-OPENSSL-5821142

Dockerfile

@@ -13,7 +13,7 @@ RUN set -ex \
         -o /go/bin/sing-box \
         -ldflags "-X \"github.com/sagernet/sing-box/constant.Version=$VERSION\" -s -w -buildid=" \
         ./cmd/sing-box
-FROM alpine AS dist
+FROM alpine:3.18.3 AS dist
 LABEL maintainer="nekohasekai <[email protected]>"
 RUN set -ex \
     && apk upgrade \