update alpine version (#5)

Author: Benjamin Blattberg

Dockerfile

@@ -5,7 +5,7 @@ WORKDIR /go/src/github.com/sensu/sensu-go
 COPY . .
 RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-X github.com/sensu/sensu-go/version.Version=$APPVERSION" -o _output/sensu-backend ./cmd/sensu-backend
 
-FROM alpine:3.6
+FROM alpine:3.12
 ENV USER=sensu
 COPY --from=build-env /go/src/github.com/sensu/sensu-go/_output/sensu-backend /usr/local/bin/sensu-backend
 RUN apk add --no-cache --update ca-certificates && \