docker: add git_url build-arg

Dockerfile

@@ -14,13 +14,14 @@ ENV GODEBUG netdns=cgo
 # image to be built from a specified Git state.  The default image
 # will use the Git tip of master by default.
 ARG checkout="master"
+ARG git_url="https://github.com/lightningnetwork/lnd"
 
 # Install dependencies and build the binaries.
 RUN apk add --no-cache --update alpine-sdk \
     git \
     make \
     gcc \
-&&  git clone https://github.com/lightningnetwork/lnd /go/src/github.com/lightningnetwork/lnd \
+&&  git clone $git_url /go/src/github.com/lightningnetwork/lnd \
 &&  cd /go/src/github.com/lightningnetwork/lnd \
 &&  git checkout $checkout \
 &&  make release-install