Fix Docker build SECRET_KEY_BASE validation error during asset precompilation

[Copilot]

The Docker build was failing during asset precompilation with the error:

ArgumentError: `secret_key_base` for production environment must be a type of String`

This occurred because the Rails application has validation that requires SECRET_KEY_BASE to be a properly formatted string in production environment. The existing approach used SECRET_KEY_BASE_DUMMY=1, which only provided a 1-character value that failed Rails 7.x validation requirements.

Changes Made

Dockerfile: Updated both asset precompilation commands to generate proper 128-character hexadecimal secret keys:

# Before
RUN SECRET_KEY_BASE_DUMMY=1 ./bin/rails assets:precompile

# After  
RUN SECRET_KEY_BASE=$(ruby -e "require 'securerandom'; puts SecureRandom.hex(64)") ./bin/rails assets:precompile

docker/README.md: Added build notes explaining the fix and Rails requirements.

Security & Safety

• Generated keys are only used during build-time asset precompilation
• Keys are not persisted in the final Docker image
• Fresh keys are generated for each build
• Runtime SECRET_KEY_BASE is still provided via environment variables as before

This fix allows the Docker build process to complete successfully while maintaining security best practices.

Fixes #330.

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