Add Dockerfile

Dockerfile

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+FROM docker.io/golang:alpine as builder
+
+COPY . /src
+WORKDIR /src
+
+RUN apk add --no-cache --update go gcc g++
+RUN go build -o /src/yggmail ./cmd/yggmail
+
+FROM docker.io/alpine
+COPY --from=builder /src/yggmail /usr/bin/yggmail
+
+EXPOSE 1143/tcp
+EXPOSE 1025/tcp
+VOLUME /etc/yggmail
+
+ENTRYPOINT ["/usr/bin/yggmail", "-database=/etc/yggmail/yggmail.db"]