fix: don't use group build

Dockerfile

@@ -32,7 +32,7 @@ RUN apt-get update \
 
 # gluon build fails when running as root
 RUN useradd -m -d /build -u 100 -g 100 -o build
-USER build:build
+USER build
 # workaround for fatal: detected dubious ownership in repository at '/builds/firmware/site-ffs'
 # https://gitlab.com/gitlab-org/gitlab-runner/-/issues/29022
 RUN git config --global --add safe.directory '*'