Update Dockerfile

Dockerfile

@@ -1,11 +1,11 @@
-FROM golang:1.14-alpine as build
+FROM golang:1.15-alpine as build
 RUN apk add --no-cache gcc musl-dev tzdata
 ADD . /app
 WORKDIR /app
 RUN go test
 RUN go build
 
-FROM alpine:3.11
+FROM alpine:3.12
 RUN apk add --no-cache tzdata ca-certificates
 COPY --from=build /app/mailygo /bin/
 WORKDIR /app