Update docker.md

Had to run an additional Docker flag to get it building on the new M1 chip from Apple. Figured it would be helpful to point that out in the Docs for the less initiated users.

docs/docker.md

@@ -10,6 +10,11 @@ cd mvt
 docker build -t mvt .
 ```
 
+Optionally, you may need to specify your platform to Docker in order to build successfully (Apple M1)
+```bash
+docker build --platform amd64 -t mvt .
+```
+
 Test if the image was created successfully:
 
 ```bash