Use /bin/bash as default CMD instruction

The previous `['/bin/bash']` was not working, running the image
with `podman run -it <name>` would evaluate to:
```
> sh -c ['/bin/bash']
sh: [/bin/bash]: No such file or directory
```

Dockerfile.xcode

@@ -36,4 +36,3 @@ CMD mkdir -p /root/xcode && \
     cp -r Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1 /tmp/iPhoneOS12.4.sdk/usr/include/c++ && \
     cd /tmp && \
     tar -cJf /root/files/iPhoneSimulator12.4.sdk.tar.xz iPhoneOS12.4.sdk
-