Add dockerfile

Dockerfile

+FROM golang:1-alpine AS builder
+
+RUN apk add --no-cache git ca-certificates
+RUN wget -qO /usr/local/bin/dep https://github.com/golang/dep/releases/download/v0.4.1/dep-linux-amd64
+RUN chmod +x /usr/local/bin/dep
+
+COPY Gopkg.lock Gopkg.toml /go/src/maubot.xyz/gitlab/
+WORKDIR /go/src/maubot.xyz/gitlab
+RUN dep ensure -vendor-only
+
+COPY . /go/src/maubot.xyx/gitlab
+RUN CGO_ENABLED=0 go build -o /usr/bin/maulabbot
+
+
+FROM scratch
+
+COPY --from=builder /usr/bin/maulabbot /usr/bin/maulabbot
+COPY --from=builder /etc/ssl/certs/ /etc/ssl/certs
+
+CMD ["/usr/bin/maulabbot"]