Try mount differently.

.ci/dolphin/build-gamecube.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 docker build -t myapp:latest -f ./.ci/dolphin/Dockerfile .
-docker run -v ./:/gamecube myapp:latest /bin/bash -c ' \
+docker run -v $GITEA_WORKSPACE/:/gamecube myapp:latest /bin/bash -c ' \
   export PATH="$DEVKITPPC/bin:$PATH" && \
   echo "Inside Docker container:" && \
   ls -l && \