[LNT] Update git URL in Dockerfile

Summary:
Update LNT git repo URL in Dockerfile from the old git mirror to the
current github official repo. This will allow the Docker image to
actually work as a Python 3 service.

Reviewers: cmatthews, hubert.reinterpretcast, kristof.beyls, PrzemekWirkus, leandron

Reviewed By: leandron

Subscribers: llvm-commits, MatzeB, leandron, PrzemekWirkus

Differential Revision: https://reviews.llvm.org/D73611

docker/Dockerfile

@@ -3,7 +3,7 @@ FROM python:alpine
 RUN apk update \
   && apk add --no-cache --virtual .build-deps git g++ postgresql-dev yaml-dev \
   && apk add --no-cache libpq \
-  && git clone https://git.llvm.org/git/lnt /var/src/lnt \
+  && git clone https://github.com/llvm/llvm-lnt /var/src/lnt \
   && python3 /var/src/lnt/setup.py install --server \
   && rm -rf /var/src \
   && apk --purge del .build-deps \