fake `SECRET_KEY_BASE` for `assets:precompile`
fake `SECRET_KEY_BASE` for `assets:precompile`

When building a docker image we precompile assets. Recent changes
require SECRET_KEY_BASE to be defined for asset compilation (?)

See: https://github.com/rails/rails/issues/32947

This inserts a dummy SECRET_KEY_BASE for this compilation pass, and should have no adverse effect.

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