[docker] fix for devbox docker image root-user build (#1811)

## Summary
This fix is due to customer message in discord mentioning the launcher
having old version.
Nix installation in `docker-image-build-publish` workflow was failing
for root-user mode.
This PR fixes it.
The fix was based on recommendation from
LnL7/nix-docker#41 (comment)

## How was it tested?
https://github.com/jetpack-io/devbox/actions/runs/7893709911

internal/devbox/generate/tmpl/DevboxImageDockerfileRootUser

@@ -5,7 +5,8 @@ RUN apt-get update
 RUN apt-get -y install bash binutils git xz-utils wget sudo
 
 # Step 2: Installing Nix
-RUN wget --output-document=/dev/stdout https://nixos.org/nix/install | sh -s -- --daemon
+RUN mkdir -p /etc/nix/
+RUN echo "filter-syscalls = false" >> /etc/nix/nix.conf && wget --output-document=/dev/stdout https://nixos.org/nix/install | sh -s -- --daemon
 RUN . ~/.nix-profile/etc/profile.d/nix.sh
 
 ENV PATH="/root/.nix-profile/bin:$PATH"