起こったこと
ローカルで動くjqueryが、herokuにあげると動かなくなる。（モーダルが出ない）
ググってアセットプレコンパイル周りが怪しいと言われるが、
それらをいじっても解決しない。
原因と解決策
application.jsの中で
jqueryを先に読み込まないとダメだった。
修正前（下3行を上に持ってきた。）
// This is a manifest file that'll be compiled into application.js, which will include all the files
// listed below.
//
// Any JavaScript/Coffee file within this directory, lib/assets/javascripts, or any plugin's
// vendor/assets/javascripts directory can be referenced here using a relative path.
//
// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
// compiled file. JavaScript code in this file should be added after the last require_* statement.
//
// Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
// about supported directives.
//
//= require rails-ujs
//= require activestorage
//= require turbolinks
//= require_tree .
ここから下3行を
//= require jquery3
//= require popper
//= require bootstrap-sprockets
修正後
// This is a manifest file that'll be compiled into application.js, which will include all the files
// listed below.
//
// Any JavaScript/Coffee file within this directory, lib/assets/javascripts, or any plugin's
// vendor/assets/javascripts directory can be referenced here using a relative path.
//
// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
// compiled file. JavaScript code in this file should be added after the last require_* statement.
//
// Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
// about supported directives.
//
ここに入れた
//= require jquery3
//= require popper
//= require bootstrap-sprockets
//= require rails-ujs
//= require activestorage
//= require turbolinks
//= require_tree .
herokuにデプロイし直して無事完了。