Server Error (500) when starting my dockerized laravel app

I have created a laravel app and then created a Dockerfile:

FROM php:7.4-cli

RUN apt-get update -y && apt-get install -y libmcrypt-dev

RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
RUN docker-php-ext-install pdo 
#mbstring

WORKDIR /app
COPY . /app

RUN composer install

EXPOSE 8000
CMD php artisan serve --host=0.0.0.0 --port=8000

Then I ran sudo docker build -t myApp . and sudo docker run -it -p 8000:8000 news-organizer. Everything worked fine.

When I copy this folder (with Dockerfile) to another location and run composer update --ignore-platform-reqs, sudo docker build --no-cache -t theApp . and sudo docker run -it -p 8888:8888 theApp the Web App starts. When I enter 127.0.0.1:8888 I get the 500 Error.

I already set all rights to sudo chmod 755 -R <myLaravelFolder>. I also tried setting different port numbers. The Dockerfile of the new folder is:

FROM php:7.4-cli

RUN apt-get update -y && apt-get install -y libmcrypt-dev

RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
RUN docker-php-ext-install pdo 
#mbstring

WORKDIR /app
COPY . /app

RUN composer install

EXPOSE 8888
CMD php artisan serve --host=0.0.0.0 --port=8888

I just can't find a way to fix the 500 Error. What can I do?

My basic idea is: Creating a Laravel Web-App. Then creating a Dockerfile and upload it somewhere. Then others can download the Web App, install Docker, and run sudo docker build -t <myImage> . and sudo docker run -it -p 8000:8000 <myImage>. With that they can use my Laravel App on their computers as docker container.

I run xubuntu 20.04 as OS.

via Chebli Mohamed