In general: if you are seeing that message you have not configured your instance to be in production mode. The warning is there because certain protections and checks are not enabled in development mode, and it should not be used like that in production.
When running in production mode, the message will automatically disappear. You can start the server in production mode by running
bundle exec rails s -e production. If you are using the docker set-up, there is a config file which allows you to set a so-called environment. Change it to
production and it should do the same. Note that settings like where files are stored and which database credentials to use are also distinct per environment.
If despite all the above you still want to forcibly remove the message:
Do not do this for production instances!
In the file `app/views/layouts/_header.html.erb` ~ line 282-288, remove the block of code which renders it:
<% if Rails.env.development? %>
<div class="notice is-danger is-banner has-padding-2 notice__dev-mode has-color-red-900">
<p>This site is in development mode!</p>
<% end %>