Update Dockerfile to git clone instead of COPY (ultralytics#7053)

Resolves git command errors that currently happen in image, i.e.:

```bash
root@382ae64aeca2:/usr/src/app# git pull
Warning: Permanently added the ECDSA host key for IP address '140.82.113.3' to the list of known hosts.
[email protected]: Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
```

Author: glenn-jocher

Dockerfile

@@ -19,7 +19,8 @@ RUN mkdir -p /usr/src/app
 WORKDIR /usr/src/app
 
 # Copy contents
-COPY . /usr/src/app
+RUN git clone https://github.com/ultralytics/yolov5 /usr/src/app
+# COPY . /usr/src/app
 
 # Downloads to user config dir
 ADD https://ultralytics.com/assets/Arial.ttf /root/.config/Ultralytics/