ssh-agent for devcontainer build

[JeanPoll]

How can I use my local git credentials in the installation phase of the devcontainer ?

in my devcontainer.json I can add:
"build": { "options": [ "--ssh=default" ] }
which results in:
docker buildx build ... --ssh=default
which is fine, but in this case the shell script needs to be executed with:
RUN --mount=type=ssh
but its not the case:
STEP 6/10: RUN cd /tmp/build-features/0 && chmod +x ./devcontainer-features-install.sh && ./devcontainer-features-install.sh

Can I somehow modify the RUN command in the build phase ?
Or do you have other ideas to share te git credentials from the host into the building phase of the container.