Using Docker run options to override settings in the Dockerfile

Usually, we first define the Dockerfile file, and then build the image file through the docker build command. Then, you can start a container instance based on the image file through docker run.

Then when starting a container, you can change some parameters in the image file, and these parameters in the image file are often defined by the Dockerfile file.

But not all definitions in the Dockerfile file can be redefined when starting the container. The Dockerfile instructions that cannot be overwritten by docker run are as follows:

• FROM
• MAINTAINER
• RUN
• ADD
• COPY

1. Overwrite the ENTRYPOINT instruction

The ENTRYPOINT instruction in the Dockerfile file is used to give the default entry point after the container is started.
The ENTRYPOINT instruction gives the default behavior after the container is started. It is generally difficult to override when starting the container, but command parameters can be appended. Here is an example:

• docker run --entrypoint /bin/bash ... , gives the subsequent command parameters of the container entry
• docker run --entrypoint="/bin/bash ..." ... , gives a new shell to the container
• docker run -it --entrypoint="" mysql bash , reset the container entry

2. Overwrite CMD command

The CMD instruction in the Dockerfile file gives the default instructions to be executed after the container is started.

When starting the container, you can set new command options for docker run to overwrite the CMD instruction in the Dockerfile file (the CMD instruction in the Dockerfile file will no longer be consulted). Here is an example:

• docker run ... <New_Command> , you can give a different command to override the default instructions in the Dockerfile
  

If the ENTRYPOINT instruction is also declared in the Dockerfile file, the above instructions will be appended to the ENTRYPOINT instruction as parameters.

3. Override the EXPOSE instruction

The EXPOSE instruction in the Dockerfile file is used to reserve ports for the host where the container is located.

Apparently this is a feature of the runtime container, so docker run can conveniently override that instruction. Here is an example:

• docker run --expose="port_number:port_number"
• docker run -p port_number:port_number/tcp , open the specified range of ports
• docker run --link="another_container_id" , link to another container
• docker run -P , open all ports

4. Override ENV directive

The ENV instruction in the Dockerfile file is used to set the environment variables in the container.

When starting a container, the following environment variables are automatically set for the container:

• HOME, set the user's home directory based on USER
• HOSTNAME, the default container hostname
• PATH, default:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
• TERM, default xterm, if the container is assigned a pseudo-TTY

docker run can conveniently override this instruction. Here is an example:

docker run -e "key=value" ... , set a new environment variable key
docker run -h ... , overwrite HOSTNAME
docker run ubuntu /bin/bash -c export

declare -x HOME="/"
declare -x HOSTNAME="85bc26a0e200"
declare -x OLDPWD
declare -x PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
declare -x PWD="/"
declare -x SHLVL="1"
declare -x deep="purple"

Setting or overriding environment variables via scripts

5. Override the VOLUME instruction

The VOLUME instruction in the Dockerfile is used to set data volumes for the container.

• docker run -v ...

• docker run -volumes-from ...

6. Overwrite USER command

The default user inside the container is root (uid=0).
In the Dockerfile file, you can specify other users as the default user of the container through USER.

• docker run -u="" ...

• docker run --user="" ...

Docker run supports the following forms of -u:

• user
• user:group
• uid
• uid:gid
• user:gid
• uid:group

7. Override the WORKDIR directive

The WORKDIR instruction in the Dockerfile file is used to set the working directory for subsequent instructions.

If the path does not exist, it is created even if it is not used in subsequent instructions.

In one, multiple WORKDIRs can exist. For relative paths, subsequent instructions inherit from the previous instruction.

In WORKDIR, you can reference previously defined environment variables.

• docker run -w="" ...

• docker run --workdir="" ...

Reference Links:

https://docs.docker.com/engine/reference/run/

https://docs.docker.com/engine/reference/builder/

Summarize

The above is the full content of this article. I hope that the content of this article will have certain reference learning value for your study or work. Thank you for your support of 123WORDPRESS.COM. If you want to learn more about this, please check out the following links