Docker-compose command doesn't override Dockerfile CMD

Question

Docker-compose reference says:

command

Override the default command.

My Dockerfile

FROM adoptopenjdk/maven-openjdk8

RUN mkdir /config

COPY /src/main/resources/application.properties /config/application.properties
COPY /src/main/resources/logback.xml /config/logback.xml

# copy wait-for-it-sh tool to sync container starting
COPY wait-for-it.sh /
RUN chmod +x /wait-for-it.sh

VOLUME /tmp

ADD target/my-svc-1.jar app.jar

RUN sh -c 'touch /app.jar'

ENV JAVA_OPTS=""

CMD [ "sh", "-c", "java $JAVA_OPTS -Djava.security.egd=file:/dev/./urandom -jar /app.jar" ]

Interesting part of my docker-compose.yml

version: '3'

services:
  ..
  my-svc:
    container_name: my-svc-container
    build:
      context: ./my-svc-folder
      dockerfile: Dockerfile
    ports:
      - "9100:9100"
      - "5005:5005"
    depends_on:
      - db
    command: ["./wait-for-it.sh", "my-svc-dbserver:3306", "--", "sh", "-c", "java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=5005 -jar /app.jar"]
  ...

Output of docker-compose build:

Building my-svc
Step 1/11 : FROM adoptopenjdk/maven-openjdk8
 ---> b3ad935008a1
Step 2/11 : RUN mkdir /config
 ---> Using cache
 ---> 5db347c13050
Step 3/11 : COPY /src/main/resources/application.properties /config/application.properties
 ---> Using cache
 ---> ade3653a018a
Step 4/11 : COPY /src/main/resources/logback.xml /config/logback.xml
 ---> Using cache
 ---> 94277fd95be1
Step 5/11 : COPY wait-for-it.sh /
 ---> Using cache
 ---> 80ac64c7c994
Step 6/11 : RUN chmod +x /wait-for-it.sh
 ---> Using cache
 ---> 1a0c99bc10ad
Step 7/11 : VOLUME /tmp
 ---> Using cache
 ---> 81864f3c9fda
Step 8/11 : ADD target/my-svc-1.jar app.jar
 ---> Using cache
 ---> 56cc5c149e37
Step 9/11 : RUN sh -c 'touch /app.jar'
 ---> Using cache
 ---> 9db3f5d983cc
Step 10/11 : ENV JAVA_OPTS=""
 ---> Using cache
 ---> a5c4828b6539
Step 11/11 : CMD [ "sh", "-c", "java $JAVA_OPTS -Djava.security.egd=file:/dev/./urandom -jar /app.jar" ]
 ---> Using cache
 ---> cb1fd876d8d6
Successfully built cb1fd876d8d6
Successfully tagged src_my-svc:latest

So, the last step is shown as CMD [ "sh", "-c", "java $JAVA_OPTS -Djava.security.egd=file:/dev/./urandom -jar /app.jar" ], the one that is in Dockerfile instead of the one in docker-compose.

What am I missing here?

Answer 1

Overriding takes place at runtime, which is when you create a container based on an image and you then start it. That last step you see is during the building phase of the actual image which (correctly) follows the instructions of your Dockerfile.

So, if you continue with docker-compose up, the container that will be created and started, will have the "overridden" configuration provided in the docker-compose.yaml file.

---

From Overriding Dockerfile image defaults

Overriding Dockerfile image defaults

When a developer builds an image from a Dockerfile or when she commits it, the developer can set a number of default parameters that take effect when the image starts up as a container. Four of the Dockerfile commands cannot be overridden at runtime: FROM, MAINTAINER, RUN, and ADD. Everything else has a corresponding override in docker run.