Styjun

I build a configuration with docker-compose :


db:
build:
context: ./context
dockerfile: /path/Dockerfile
image: mariadb:dev


In Dockerfile :


FROM mariadb:latest
MAINTAINER Billy
COPY ./ /var/lib/mysql


After launch docker-compose up i have:


REPOSITORY TAG
mariadb dev
mariadb latest


Why i have a image mariadb:latest and not only mariadb:dev ?

Thanks you for your help (i am a noob in docker and sorry for my english)

db:
 build:
 context: ./context
 dockerfile: /path/Dockerfile
 image: mariadb:dev

This tells Docker that you want to build an image and call it mariadb and tag it as dev (=mariadb:dev).

Your Dockerfile is based on the image mariadb with its tag latest (FROM mariadb:latest).

So Docker first has to pull mariadb:latest from the docker hub. After that step this image is in your local registry. After building your image (mariadb:dev) this image is also in your local registry.

This is why you have both in your registry. mariadb:latest is the base image for your image. So Docker has to pull that image in the first step of your Dockerfile. And so it is in your registry.

That behaviour is documented in the official Docker docs

If you specify image as well as build, then Compose names the built
image with the webapp and optional tag specified in image:

build: ./dir 
image: webapp:tag

This results in an image named webapp
and tagged tag, built from ./dir.

I think what you really want is to use the mariadb:latest image from the docker hub and include your modified files as a volume instead of building a new image.

This would look like something like the follwing in your docker-compose.yml (please refer to the docs of the maria db image on how to use it in detail):

db:
 image: mariadb:latest
 volumes:
 - "<relativePath>:/var/lib/mysql"
 environment:
 - MYSQL_ROOT_PASSWORD=password

where <relativePath> is the relative path from the location where your docker-compose.yml is located / where you execute the docker-compose up. You could for example create a sub folder "mysql" containing all the files you want to be mounted to /var/lib/mysql inside the container and then use - "mysql:/var/lib/mysql". (By using . instead you will mount docker-compose.yml and everything inside the same folder into /var/lib/mysql inside the container)

You have both those images because mariadb:latest is an intermediate image for your custom image - it's necessary to have it before executing your two custom layers (MANTAINER and COPY, respectively).

Think of your custom image as a stack of images:

• The mariadb:latest image from Docker Hub (I assume) is stacked first


• A MANTAINER intermediate layer (MANTAINER Billy) comes next


• A COPY intermediate layer (COPY ./ /var/lib/mysql) comes last

Then, docker creates a tag (mariadb:dev) and points it to your newly created image.

So, more specifically, Docker needs to have mariadb:latest in its local cache to make it possible for your image to be built.

You can find more information about this in the official Docker documentation.