Try something else?

.ci/dolphin/build-gamecube.sh

@@ -1,9 +1,9 @@
 #!/bin/bash
 docker build -t myapp:latest -f ./.ci/dolphin/Dockerfile .
-docker run -v ./:/workdir myapp:latest /bin/bash -c ' \
+docker run -v ./:/gamecube myapp:latest /bin/bash -c ' \
   export PATH="$DEVKITPPC/bin:$PATH" && \
   ls -l && \
-  cd /workdir && \
+  cd /gamecube && \
   ls -l && \
   rm -rf build-gamecube && \
   mkdir -p build-gamecube && \