環境

PHP : 7.1.3
Laravel : 5.7
docker-compose.yml

# docker-compose.ymlのバージョン
version: '3'
# docker volumeの設定
volumes:
  docker-volume:

# services以下に各コンテナの設定を書く
services:
  #Webサーバーのコンテナ
  web:
    image: nginx:1.18
    ports:
    - '8000:80'
    volumes:
    - ./server:/var/www
    - ./docker/nginx/default.conf:/etc/nginx/conf.d/default.conf
    depends_on:
    - php

  #アプリケーションのコンテナ
  php:
    build: ./docker/php
    volumes:
    - ./server:/var/www

  #データベースのコンテナ
  db:
    image: mysql:5.7
    ports:
    - '13306:3306'
    environment:
      MYSQL_DATABASE: laravel
      MYSQL_USER: root
      MYSQL_ROOT_PASSWORD: password
      TZ: 'Asia/Tokyo'
    volumes:
    - docker-volume:/var/lib/mysql

エラー内容

xxx@MacBook-Pro ponshuki % git push heroku master
Enumerating objects: 1653, done.
Counting objects: 100% (1653/1653), done.
Delta compression using up to 8 threads
Compressing objects: 100% (1600/1600), done.
Writing objects: 100% (1653/1653), 6.16 MiB | 2.15 MiB/s, done.
Total 1653 (delta 1033), reused 0 (delta 0), pack-reused 0
remote: Compressing source files... done.
remote: Building source:
remote: 
remote: -----> Building on the Heroku-20 stack
remote:  !     No default language could be detected for this app.
remote: 			HINT: This occurs when Heroku cannot detect the buildpack to use for this application automatically.
remote: 			See https://devcenter.heroku.com/articles/buildpacks
remote: 
remote:  !     Push failed
remote: Verifying deploy...
remote: 
remote: !	Push rejected to rocky-ravine-27668.
remote: 
To https://git.heroku.com/rocky-ravine-27668.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/rocky-ravine-27668.git'
xxx@MacBook-Pro ponshuki %  git status
On branch master
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
	modified:   server/.env.example

Untracked files:
  (use "git add <file>..." to include in what will be committed)
	Procfile

no changes added to commit (use "git add" and/or "git commit -a")

原因

ディレクトリ構造が問題だった
dockerを使用していたため、直下のデイレクトリにcomposer.jsonがなくてはいけない。
↓
phpをherokuが認識できない

解決方法

git subtree push --prefix server/ heroku master

できたー！