Dockerfile preprocessor should respect current user when mounting secrets

[NiklasRosenstein]

The Dockerfile preprocessing injecting mounts for secrets does not take into account the current user for the RUN step.

USER shane
RUN cat /etc/secrets/MYSECRET

When passing a build secret to build_docker_image(), the preprocessing step will change the Dockerfile to

USER shane
RUN --mount=type=secret,id=MYSECRET cat /etc/secrets/MYSECRET

But it should actually be

  USER shane
- RUN --mount=type=secret,id=MYSECRET cat /etc/secrets/MYSECRET
+ RUN --mount=type=secret,id=MYSECRET,uid=1000 cat /etc/secrets/MYSECRET

However, this has two issues:

• How does Kraken know the user ID of the user if the USER block uses a name rather than an ID? (It's not guaranteed to be 1000)
• How does Kraken know the ID of the user if the base image is already switching to a different user?