s6-overlay-$ARCH.tar.gz file is not removed

[e-minguez]

octoprint-docker/Dockerfile

Line 18 in 978b7fe

&& curl -fsSLO "https://github.com/just-containers/s6-overlay/releases/download/$S6_VERSION/s6-overlay-$ARCH.tar.gz"

Similar to #276

In this case I'm not sure why the build process is split in two different containers tho.