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.javascript

@@ -26,4 +26,4 @@ RUN git clone https://github.com/mono/mono --branch mono-${mono_version} --singl
     cd .. && \
     rm -rf /root/mono /root/godot-mono-builds
 
-CMD ['/bin/bash']
+CMD /bin/bash