feat: Add dockerfile (#18)

Dockerfile

@@ -0,0 +1,26 @@
+#### development stage
+FROM golang:1.16-buster AS builder
+
+# set envvar
+ENV CGO_ENABLED=0
+ENV GOOS=linux
+ENV GO111MODULE='on'
+
+# set workdir
+WORKDIR /code
+
+# get project dependencies
+COPY go.mod go.sum /code/
+RUN go mod download
+
+# copy files
+COPY . /code
+
+# generate binary
+RUN go build -ldflags="-s -w" -o ./app ./cmd/postmand
+
+#### final stage
+FROM gcr.io/distroless/base:nonroot
+COPY --from=builder /code/app /
+COPY --from=builder /code/db/migrations /db/migrations
+ENTRYPOINT ["/app"]