Fix pip install line in Dockerfile

Summary:
The pip install line in Dockerfile is missing the -r switch and is
not executed from the directory where the requirement file is, both of
which make the docker image fail to build. This commit fixes those
issues and also simplify the line by using pip3 instead of
python3 -m pip.

Reviewed By: cmatthews

Differential Revision: https://reviews.llvm.org/D79238
diff --git a/docker/Dockerfile b/docker/Dockerfile
index b16e195..c193ad9 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -4,7 +4,7 @@
   && apk add --no-cache --virtual .build-deps git g++ postgresql-dev yaml-dev \
   && apk add --no-cache libpq \
   && git clone https://github.com/llvm/llvm-lnt /var/src/lnt \
-  && python3 -m pip install /var/src/lnt/requirements.server.txt \
+  && cd /var/src/lnt && pip3 install -r requirements.server.txt \
   && rm -rf /var/src \
   && apk --purge del .build-deps \
   && mkdir /var/log/lnt