Using secrets with docker is fine but fails for docker-compose

Question

I have a Dockerfile that uses secrets and I can successfully build the image using docker build. However, when I try to build the same image using docker-compose build I get the error:

ERROR: Dockerfile parse error line 4: Unknown flag: mount

This occurs on Ubuntu 20.04LTS (Docker version 18.09.6, build 481bc77, docker-compose version 1.20.0-rc2, build 8c4af54).

On RHEL 7.9 (Docker version 20.10.7 build f0df350, docker-compose version 1.29.2, build 5becea4c) a different error occurs:

[2/2] RUN --mount=type=secret,id=the_secret cat /run/secrets/the_secret:  
 #8 0.466 cat: /run/secrets/the_secret: No such file or directory

How can I use docker-compose to build my images that involve secrets?

Build using docker (works)

#!/bin/bash
export COMPOSE_DOCKER_CLI_BUILD=1
export DOCKER_BUILDKIT=1


echo "I have a secret" > secret.txt
docker build --secret id=the_secret,src=./secret.txt .

build using docker-compose (fails)

export COMPOSE_DOCKER_CLI_BUILD=1
export DOCKER_BUILDKIT=1


echo "I have a secret" > secret.txt
docker-compose build --no-cache test

Dockerfile

# syntax=docker/dockerfile:1.2
FROM python:3.8

RUN --mount=type=secret,id=the_secret cat /run/secrets/the_secret

docker-compose.yml

version: "3.6"

services:
    test:
        build: .
        secrets:
            - the_secret

secrets:
    the_secret:
        file: secret.txt

Semantics

Your Dockerfile needs the syntax: # syntax=docker/dockerfile:experimental. – Crescen

@ObsidianAge the docker documentation uses the dockerfile:1.2 syntax specification at docs.docker.com/develop/develop-images/build_enhancements so that should not be the problem. Same problem arises when I change it as you suggest. – Semantics

Just in case people come here looking for an answer, the syntax here looks nearly correct. The answer in the link below uses context: . rather than build, but a fix for this in docker-compose came in April 2022, so you need to be using docker-compose 2.5.0+: - #72281271 The first answer in the above links out to the PRs in GitHub. – Gauguin