ci: build-oci-image: create entrypoint script and put env sources in it

.docker/Dockerfile

@@ -31,4 +31,6 @@ USER gentooligan
 
 WORKDIR /workspace
 
-ENTRYPOINT /bin/bash
+ENTRYPOINT ["/entrypoint.sh"]
+
+CMD ["/bin/bash"]

.docker/entrypoint.sh

@@ -0,0 +1,6 @@
+#!/usr/bin/env sh
+
+
+source /etc/profile
+source /lib/gentoo/functions.sh
+exec "$@"