How to pass dynamic build args via Compose file

Question

I am currently building a container in my compose with a command

docker compose build --build-arg VAR_NAME=$(some_command) service_name

and I would like to build the same container with the simpler command docker compose build service_name, but I have not found a way to have docker compute the new output of some_command at each build.

To accomplish this I can freely change both the compose file and the Dockerfile for the container.

Is there any way to have docker instantiate a build argument or some other kind of variables dynamically on each build?

What I tried so far:

My first idea was to do something like the following

services:
    service_name:
        build:
            context: ./service_name
            args:
                VAR_NAME: $(some_command)

or

                VAR_NAME: "$(some_command)"

or

                VAR_NAME: "$$(some_command)"

but none of them works, even if

services:
    service_name:
        build:
            context: ./service_name
            args:
                VAR_NAME: 'example of some_command output'

works correctly.

i don't think you can set build args dynamically in a docker-compose file. i use build scripts for this. for example build.sh contains docker-compose build --build-arg MY_VAR=$(command) — tom
– tom, Commented Sep 12, 2022 at 11:52
How to set uid and gid in Docker Compose? discusses a similar problem (trying to set user: $(id -u)) and notes that you cannot do command substitution or other advanced shell substitutions in a Compose file. — David Maze
– David Maze, Commented Sep 12, 2022 at 12:52

tom · Accepted Answer · 2022-09-12 12:02:39Z

4

it seems that docker does not support running shell commands in a docker-compose.yml

see the Using shell command in docker-compose.yml github discussion

you could use a script to wrap your build command.

# build.sh

docker-compose build --build-arg MY_VAR=$(command)

answered Sep 12, 2022 at 12:02

tom

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2 Comments

afiori Over a year ago

In the future I might have containers that use similar build args with different per-container values. I guess I will have to rename them.

Ariel Over a year ago

It looks heavy but it does work, thanks.

afiori · Accepted Answer · 2022-09-12 12:31:05Z

0

@tom's answer is likely better but out of stubbornness I decided to generate a docker-compose.ovverride.yml with the correct values.

I went for ad-hoc template processed with sed:

docker-compose.template:

services:
    admin:
        build:
            args:
                VAR_NAME: __VAR_NAME__
                VAR_NAME2: __VAR_NAME2__

and a simple sed script:

sed \
  -e "s/__VAR_NAME__/$(some_command)/g" \
  -e "s/__VAR_NAME2__/$(some_command2)/g" \
  ./docker-compose.template \
  > ./docker-compose.override.yml

This is quite ugly but works for my use-case

TODO:

Fix escaping in sed's regex pattern, currently the output is in the pattern [0-9]+ so it is fine
Use a better template language

answered Sep 12, 2022 at 12:31

afiori

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