Unsupported config option for services.web: 'dockerfile' - docker-compose

Question

I am setting up a new environment with php and mysql in docker. i am using docker-compose file. while installing i realize that i need few more PHP extentensions. I have gone through suggestion online where it is suggested to write a docker file and call it in docker-compose.yml. it is alwasy showing below error
Unsupported config option for services.web: 'dockerfile'
please find docker-compose.yml and Dockerfile below
docker-compose.yml
version: '3'
services:
db:
image: mysql:5.7
environment:
MYSQL_ROOT_PASSWORD: asdf
# MYSQL_DATABASE: test_db
# MYSQL_USER: root
# MYSQL_PASSWORD: asdf
volumes:
- /var/mysql/:/var/lib/mysql
ports:
- "3306:3306"
web:
# image: alankar1985/php7.2:apache2
container_name: php_web
dockerfile: Dockerfile
# command: apt-get install -y php-gd
depends_on:
- db
volumes:
- /var/www/html/:/var/www/html/
ports:
- "80:80"
stdin_open: true
tty: true
Dockerfile
FROM alankar1985/php7.2:apache2
RUN apt install php-gd
Unsupported config option for services.web: 'dockerfile'

Answer 1

The best way to troubleshoot these type of issues is checking the docker-compose reference for the specific version - https://docs.docker.com/compose/compose-file/.
Without testing, the issue is because you need to put dockerfile under build.
Old:
web:
dockerfile: Dockerfile
New:
web:
build:
context: .
dockerfile: Dockerfile
Since you are using the standard Dockerfile filename, you could also use:
build:
context: .
From reference page:
DOCKERFILE Alternate Dockerfile.
Compose uses an alternate file to build with. A build path must also
be specified.
build:
context: .
dockerfile: Dockerfile-alternate