Ensure asset precompilation on deploy does not fail due to missing secrets

[bshand]

Rails has deprecated the use of config/secrets.yml as a store for e.g. secret key base. This had a knock on effect of being a breaking change for asset precompilation when running deployments - secret key base was expected to be supplied either through environment variable or encrypted credentials file.

https://api.rubyonrails.org/v7.2/classes/Rails/Application.html#method-i-secret_key_base

We look for it first in ENV["SECRET_KEY_BASE"], then in credentials.secret_key_base. For most applications, the correct place to store it is in the encrypted credentials file.

Generating a random secret_key_base and storing it in tmp/local_secret.txt can also be triggered by setting ENV["SECRET_KEY_BASE_DUMMY"]. This is useful when precompiling assets for production as part of a build step that otherwise does not need access to the production secrets.

The integration test failures are a longstanding issue of brittle tests, unrelated to this commit.