Feature: repalce dep with go module

Dockerfile

@@ -2,11 +2,9 @@ FROM golang:latest as builder
 RUN wget http://geolite.maxmind.com/download/geoip/database/GeoLite2-Country.tar.gz -O /tmp/GeoLite2-Country.tar.gz && \
     tar zxvf /tmp/GeoLite2-Country.tar.gz -C /tmp && \
     cp /tmp/GeoLite2-Country_*/GeoLite2-Country.mmdb /Country.mmdb
-RUN curl https://raw.githubusercontent.com/golang/dep/master/install.sh | sh && \
-    mkdir -p /go/src/github.com/Dreamacro/clash
-WORKDIR /go/src/github.com/Dreamacro/clash
-COPY . /go/src/github.com/Dreamacro/clash
-RUN dep ensure && \
+WORKDIR /clash-src
+COPY . /clash-src
+RUN go mod download && \
     GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go build -ldflags '-w -s' -o /clash && \
     chmod +x /clash