fix: use runuser -u flag to avoid bash --command error (#49)

The previous invocation `runuser builder --command 'bash -l -c /build.sh'`
placed the username before the --command option. In su-compatible mode,
once the username is parsed, all subsequent arguments are treated as
shell arguments rather than runuser options. This caused --command to be
passed directly to bash, producing:

  bash: --command: invalid option

This started failing after Arch Linux updated util-linux to a version
with stricter argument parsing in su-compatible mode.

The fix switches to the -u/--user mode which is the recommended way to
run a command as another user, with -- separating runuser options from
the command arguments.

Author: rursache

entrypoint.sh

@@ -17,4 +17,4 @@ chown -vR builder:builder /home/builder
 chmod -vR 600 /home/builder/.ssh/*
 echo '::endgroup::'
 
-exec runuser builder --command 'bash -l -c /build.sh'
+exec runuser -u builder -- bash -l -c /build.sh