Use pipenv sync command in Dockerfile.

`sync` is recommended for deployments. It installs exactly what's in
the Pipfile.lock and does not try to update the lockfile.

https://docs.pipenv.org/en/latest/advanced/#using-pipenv-for-deployments

Dockerfile

@@ -36,7 +36,7 @@ COPY . .
 # Install app dependencies
 RUN ./script/write_dotenv && \
       pip install pipenv uwsgi && \
-      PIPENV_VENV_IN_PROJECT=1 pipenv install && \
+      PIPENV_VENV_IN_PROJECT=1 pipenv sync && \
       yarn install && \
       cp -rf ./node_modules/uswds/src/fonts ./static/ && \
       yarn build