You must set ELECTRIC_SECRET

Following instructions when I run `docker compose up` I see
```
Runtime terminating during boot ({#{message=><<"You must set ELECTRIC_SECRET unless ELECTRIC_INSECURE=true. Setting ELECTRIC_INSECURE=true risks exposing your database, only use insecure mode in development or you've otherwise secured the Electric API">>,'__struct__'=>'Elixir.RuntimeError','__exception__'=>true},[{'Elixir.Electric.Config','validate_security_config!',2,[{file,"lib/electric/config.ex"},{line,411},{error_info,#{module=>'Elixir.Exception'}}]},{elixir_eval,'__FILE__',1,[{file,"/app/releases/1.0.20/runtime.exs"},{line,172}]},{elixir,eval_external_handler,3,[{file,"src/elixir.erl"},{line,386}]},{erl_eval,do_apply,7,[{file,"erl_eval.erl"},{line,919}]},{erl_eval,exprs,6,[{file,"erl_eval.erl"},{line,271}]},{elixir,eval_forms,4,[{file,"src/elixir.erl"},{line,364}]},{'Elixir.Module.ParallelChecker',verify,1,[{file,"lib/module/parallel_checker.ex"},{line,120}]},{'Elixir.Code',validated_eval_string,3,[{file,"lib/code.ex"},{line,572}]}]})
```