You should not use su in a dockerfile, however you should use the USER instruction in the Dockerfile.
At each stage of the Dockerfile build, a new container is created so any change you make to the user will not persist on the next build stage.
For example:
RUN whoami
RUN su test
RUN whoami
This would never say the user would be test as a new container is spawned on the 2nd whoami. The output would be root on both (unless of course you run USER beforehand).
If however you do:
RUN whoami
USER test
RUN whoami
You should see root then test.
Alternatively you can run a command as a different user with sudo with something like
sudo -u test whoami
But it seems better to use the official supported instruction.