phpで作成したアプリをHerokuにデプロイできない

前提・実現したいこと

PHPで非同期通信のTodo Listアプリを作成しました。
Herokuにデプロイしたところ以下の様なエラーメッセージが表示され、デプロイが失敗してしまいます。

発生している問題・エラーメッセージ

command
1git push heroku master     Enumerating objects: 57, done.
2Counting objects: 100% (57/57), done.
3Delta compression using up to 4 threads
4Compressing objects: 100% (50/50), done.
5Writing objects: 100% (57/57), 31.74 KiB | 1015.00 KiB/s, done.
6Total 57 (delta 11), reused 0 (delta 0)
7remote: Compressing source files... done.
8remote: Building source:
9remote: 
10remote: -----> Building on the Heroku-20 stack
11remote: -----> Using buildpack: heroku/php
12remote: -----> PHP app detected
13remote: 
14remote:  !     ERROR: No 'composer.lock' found!
15remote:  !     
16remote:  !     A 'composer.lock' file was not found in your project, but there
17remote:  !     is a 'composer.json' file with dependencies inside 'require'.
18remote:  !     
19remote:  !     The lock file is required in order to guarantee reliable and
20remote:  !     reproducible installation of dependencies across platforms and
21remote:  !     deploys. You must follow the Composer best practice of having
22remote:  !     your lock file under version control in order to deploy. The
23remote:  !     lock file must not be in your '.gitignore'.
24remote:  !     
25remote:  !     Please perform the following steps locally on your computer to
26remote:  !     resolve this issue before attempting another deploy:
27remote:  !     1) remove 'composer.lock' from file '.gitignore', if present
28remote:  !     2) if no 'composer.lock' exists, run 'composer update'
29remote:  !     3) stage the lock file changes using 'git add composer.lock'
30remote:  !     4) if you edited '.gitignore', also run 'git add .gitignore'
31remote:  !     5) commit the change using 'git commit'
32remote:  !     
33remote:  !     Please remember to always keep your 'composer.lock' updated in
34remote:  !     lockstep with 'composer.json' to avoid common problems related
35remote:  !     to dependencies during collaboration and deployment.
36remote:  !     
37remote:  !     Please refer to the Composer documentation for further details:
38remote:  !     https://getcomposer.org/doc/
39remote:  !     https://getcomposer.org/doc/01-basic-usage.md
40remote: 
41remote:  !     Push rejected, failed to compile PHP app.
42remote: 
43remote:  !     Push failed
44remote: Verifying deploy...
45remote: 
46remote: !       Push rejected to dotinstall-todolist.
47remote: 
48To https://git.heroku.com/dotinstall-todolist.git
49 ! [remote rejected] master -> master (pre-receive hook declined)
50error: failed to push some refs to 'https://git.heroku.com/dotinstall-todolist.git'

デプロイまでの流れ

デプロイまでまで、次の様な手順で実行しました。

heroku
1$ heroku login
2$ cd my-project/
3$ git init
4$ heroku git:remote -a dotinstall-todolist
5$ git add .
6$ git commit -am "make it better"
7$ git push heroku master

最後のgit heroku masterの箇所でエラーが発生してします

尚、その後、heroku openを実行すると、welcome to heroku appとだけ表示されます。