fix: specify steam user outside RUN

Dockerfile

@@ -25,7 +25,6 @@ ENV GAMEPORT=7787 \
 SHELL [ "/bin/bash", "-c" ]
 
 RUN <<__EOR__
-
 apt-get update
 
 apt-get install -y --no-install-suggests --no-install-recommends \
@@ -34,11 +33,14 @@ apt-get install -y --no-install-suggests --no-install-recommends \
     gnupg=2.2.40-1.1
 
 rm -rf /var/lib/apt/lists/*
+__EOR__
 
 # HACK: Hard coding the user to be "steam" is not great, but I'm too lazy to really do this the
 # *right* way (which is making sure the USER ends up as steam via an entry point up in the steamcmd
 # image). For now, this will have to do.
-su steam -c "/bin/bash" <<-__EOS__
+USER steam
+
+RUN <<__EOR__
 if (( use_squad_beta == 1 )); then
     # Install Squad from the Beta branch
     "${STEAM_CMD_INSTALL_DIR}/steamcmd.sh" \
@@ -56,8 +58,6 @@ else
         +app_update ${steam_app_id} validate \
         +quit
 fi
-__EOS__
-
 __EOR__
 
 FROM build-squad AS mods