Allow assets:precompile to be run in a production build step without passing in RAILS_MASTER_KEY (#46760)

* Add 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.

* Test SECRET_KEY_BASE_DUMMY

railties/CHANGELOG.md

@@ -1,3 +1,15 @@
+*   Add ENV["SECRET_KEY_BASE_DUMMY"] for starting production environment with a generated secret base key,
+    which can be used to run tasks like `assets:precompile` without making the RAILS_MASTER_KEY available
+    to the build process.
+    
+    Dockerfile layer example:
+
+    ```
+    RUN SECRET_KEY_BASE_DUMMY=1 bundle exec rails assets:precompile
+    ```
+
+    *DHH*
+
 *   Show descriptions for all commands in Rails help
 
     When calling `rails help` most commands missed their description. We now

railties/lib/rails/application.rb

@@ -461,11 +461,17 @@ def secrets
     # In development and test, this is randomly generated and stored in a
     # temporary file in <tt>tmp/development_secret.txt</tt>.
     #
+    # You can also set <tt>ENV["SECRET_KEY_BASE_DUMMY"]</tt> to trigger the use of a randomly generated
+    # secret_key_base that's stored in a temporary file. This is useful when precompiling assets for
+    # production as part of a build step that otherwise does not need access to the production secrets.
+    #
+    # Dockerfile example: <tt>RUN SECRET_KEY_BASE_DUMMY=1 bundle exec rails assets:precompile</tt>.
+    #
     # In all other environments, we look for it first in <tt>ENV["SECRET_KEY_BASE"]</tt>,
     # then +credentials.secret_key_base+, and finally +secrets.secret_key_base+. For most applications,
     # the correct place to store it is in the encrypted credentials file.
     def secret_key_base
-      if Rails.env.development? || Rails.env.test?
+      if Rails.env.development? || Rails.env.test? || ENV["SECRET_KEY_BASE_DUMMY"]
         secrets.secret_key_base ||= generate_development_secret
       else
         validate_secret_key_base(

railties/test/application/configuration_test.rb

@@ -762,6 +762,20 @@ def index
       assert_match(/Missing `secret_key_base`./, error.message)
     end
 
+    test "dont raise in production when dummy secret_key_base is used" do
+      ENV["SECRET_KEY_BASE_DUMMY"] = "1"
+
+      app_file "config/initializers/secret_token.rb", <<-RUBY
+        Rails.application.credentials.secret_key_base = nil
+      RUBY
+
+      assert_nothing_raised do
+        app "production"
+      end
+    ensure
+      ENV["SECRET_KEY_BASE_DUMMY"] = nil
+    end
+
     test "raise when secret_key_base is not a type of string" do
       add_to_config <<-RUBY
         Rails.application.credentials.secret_key_base = 123