Merge pull request #118 from messagebird/go-118-alpine-315

upgrade Docker container to use golang:1.18 and alpine:3.15. Pin alpine

Author: marcel corso gonzalez

Dockerfile

@@ -1,12 +1,12 @@
-FROM golang:1.17 AS builder
+FROM golang:1.18 AS builder
 
 WORKDIR /build
 
 COPY . .
 
 RUN GO111MODULE=on CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -mod vendor -o sachet github.com/messagebird/sachet/cmd/sachet
 
-FROM alpine
+FROM alpine:3.15
 
 COPY --from=builder /build/sachet /usr/local/bin
 COPY --chown=nobody examples/config.yaml /etc/sachet/config.yaml