Format strips `--from` directive in Dockerfile

Running on a recent Emacs under Linux x86_64, I notice formatting on save breaks the Dockerfile. 

The formatting strips out commands for multi-stage builds: 
```
COPY --from=builder /go/src/app/bin/foo /usr/bin
```
gets reformatted to: 
```
COPY /go/src/app/bin/foo /usr/bin
```

This breaks multi-stage builds relying on copying files/artifacts between the phases. 