Use --chmod flag instead of separate RUN chmod command in backend Dockerfile

[mueckinger]

It is good practise to do the chmod in the COPY, not in a separate command as this creates an extra layer and is known to not work in every build environment, for example in my setup which is kaniko.

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Note

Low Risk
Low risk build change that only affects Docker image layering and file permissions for the entrypoint script. Potential impact is limited to environments/builders that differ in support for COPY --chmod.

Overview
Updates deploy/docker/backend.Dockerfile to apply executable permissions to backend-entrypoint.sh during COPY via --chmod=755, removing the separate RUN chmod +x layer.

This reduces image layers and improves compatibility with builders where post-copy chmod can be unreliable.

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