Fix ENTRYPOINT of `toolchain` image

[pat-s]

"/bin/bash" is not a valid entrypoint and will result in errors when running the container manually (I know this is usually not needed but helpful for some debugging actions).

The change allows running containers from the image.

[zeripath]

yeah it was never my intention that you would run the container manually without setting your own entrypoint directly but I guess it's helpful to get this in.

[pat-s]

Yeah I see, it's not a big issue for anyone - besides if you wanna rebuild the image and inspect some things :)