DL3003: Use WORKDIR to switch directories

Using `cd` inside RUN instructions does not persist across layers because each RUN starts in the WORKDIR. Chaining `cd dir && command` works but makes the Dockerfile harder to read and maintain. Developers often forget that `cd` in one RUN does not affect the next, which leads to path-related bugs. WORKDIR is the idiomatic way to set the working directory and it persists across all subsequent instructions.