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Docker - Use docker-compose to override CMD

Docker - Use docker-compose to override CMD

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I've been dabbling with Docker to run services on my new Raspberry Pi and being a beginner at Docker myself, it was hard sometimes to debug why a docker container was failing as soon as it was started. Since a container "stays alive" only until its CMD process is running, there is no way to do a quick look-see inside the container if it has already Exited.

To change the Dockerfile to perform trial and error means that you will have to rebuild it again - which is time consuming.

One way to keep a container running without having to do this, is to override the Dockerfile command from the docker-compose.yml file.

For example:

version: "3"
services:
  redis:
    image: redis
  web:
    image: web:v001
    build:
      context: .
      dockerfile: Dockerfile
    ports:
      - "3000:3000"
    volumes:
      - .:/opt/www
    depends_on:
      - redis

And in the Dockerfile you have a command:

FROM node:14-alpine

ENV APP_PATH /opt/www
RUN mkdir -p $APP_PATH

WORKDIR $APP_PATH

COPY package.json ./
COPY yarn.lock ./

RUN yarn

COPY . .

RUN yarn build

CMD yarn start

EXPOSE 3000

Anytime CMD yarn start fails, the container exits and is no more accessible for debugging.

Modify docker-compose.yml to include command override.

version: "3"
services:
  redis:
    image: redis
  web:
    image: web:v001
    build:
      context: .
      dockerfile: Dockerfile
    command: "tail -f /dev/null"     #   <--- <--- <---
    ports:
      - "3000:3000"
    volumes:
      - .:/opt/www
    depends_on:
      - redis

Now, when you start the container using docker-compose up -d, the container stays running.

Next, use docker ps -a to list all containers and obtain the hash for the container id.

Then, run sh within the container by: docker exec -it <sha> sh.

If everything goes well, you should see the shell prompt within the container:

/opt/www #

Now, you may debug for missing files, env vars, or even try to run yarn start and see why it is failing.

 
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