RUN cd / does absolutely nothing. WORKDIR / changes the working directory for future commands.
Each RUN command runs in a new shell and a new environment (and technically a new container, though you won’t usually notice this). The ENV and WORKDIR directives before it affect how it starts up. If you have a RUN step that just changes directories, that will get lost when the shell exits, and the next step will start in the most recent WORKDIR of the image.
FROM busybox
WORKDIR /tmp
RUN pwd # /tmp
RUN cd / # no effect, resets after end of RUN line
RUN pwd # still /tmp
WORKDIR /
RUN pwd # /
RUN cd /tmp && pwd # /tmp
RUN pwd # /
(For the same reason, RUN export doesn’t do anything that outlives the current Dockerfile instructions, and RUN . or the non-standard RUN source won’t cause environment variables to be set.)