Pin the builder stage as alpine3.18

Author: arukiidou

Dockerfile

@@ -1,5 +1,5 @@
 # Multi-stage build to generate custom k6 with extension
-FROM golang:1.20-alpine as builder
+FROM golang:1.20-alpine3.18 as builder
 WORKDIR $GOPATH/src/go.k6.io/k6
 COPY . .
 RUN apk --no-cache add git=~2