Add a Dockerfile and more configurable environment variables

Dockerfile

+FROM golang:1.14.2-alpine AS builder
+
+RUN go env -w GO111MODULE=auto \
+  && go env -w CGO_ENABLED=0 \
+  && mkdir /build
+
+WORKDIR /build
+
+COPY ./ .
+
+RUN cd /build \
+  && go build -ldflags "-s -w -extldflags '-static'" -o cqhttp
+
+FROM alpine:latest
+
+COPY --from=builder /build/cqhttp /usr/bin/cqhttp
+RUN chmod +x /usr/bin/cqhttp
+
+WORKDIR /data
+
+ENTRYPOINT [ "/usr/bin/cqhttp" ]