devcontainers using docker-compose can't find Dockerfile

Start with .devcontainer/devcontainer.json and docker-compose.yml, then reproduce the failure using the Podman/VS Code Flatpak setup described in the log. Compare the Dockerfile path seen by podman-compose with the project layout and determine whether the Flatpak sandbox is involved; done means the cause and a reproducible fix or documented workaround are established.

I have a project that has an existing docker-compose.yml that I'd like to use. This docker-compose.yml runs a service built from a local Dockerfile, referenced as follows:

build:
      dockerfile: "Dockerfile"
      context: .

My .devcontainer/devcontainer.json is as follows:

{
    "name": "<project name>",
    "dockerComposeFile": "../docker-compose.yml",
    "service": "<service in docker-compose>",
    "workspaceFolder": "/app",
    "shutdownAction": "stopCompose"
}

Opening VSCode using this seemingly simple setup however fails:

[2024-10-18T13:35:07.085Z] Traceback (most recent call last):
  File "/usr/bin/podman-compose", line 8, in <module>
[2024-10-18T13:35:07.085Z]     sys.exit(main())
             ^^^^^^
  File "/usr/lib/python3.12/site-packages/podman_compose.py", line 3504, in main
[2024-10-18T13:35:07.086Z]     asyncio.run(async_main())
  File "/usr/lib/python3.12/asyncio/runners.py", line 194, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/asyncio/runners.py", line 118, in run
[2024-10-18T13:35:07.086Z]     return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/asyncio/base_events.py", line 687, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/site-packages/podman_compose.py", line 3500, in async_main
[2024-10-18T13:35:07.086Z]     await podman_compose.run()
  File "/usr/lib/python3.12/site-packages/podman_compose.py", line 1743, in run
[2024-10-18T13:35:07.087Z]     retcode = await cmd(self, args)
              ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/site-packages/podman_compose.py", line 2422, in compose_build
    s = await t
        ^^^^^^^
  File "/usr/lib/python3.12/asyncio/tasks.py", line 631, in _wait_for_one
    return f.result()  # May raise f.exception().
           ^^^^^^^^^^
  File "/usr/lib/python3.12/site-packages/podman_compose.py", line 2373, in build_one
[2024-10-18T13:35:07.087Z]     raise OSError("Dockerfile not found in " + ctx)
[2024-10-18T13:35:07.087Z] OSError: Dockerfile not found in /home/<user>/<project>
[2024-10-18T13:35:07.115Z] Error: executing /usr/bin/podman-compose --project-name <project name> -f /home/<user>/<project>/docker-compose.yml -f /home/<user>/.var/app/com.visualstudio.code/config/Code/User/globalStorage/ms-vscode-remote.remote-containers/data/docker-compose/docker-compose.devcontainer.build-1729258506835.yml build: exit status 1
[2024-10-18T13:35:07.123Z] Stop (287 ms): Run: /home/<user>/.local/bin/podman-host compose --project-name <project name> -f /home/<user>/<project>/docker-compose.yml -f /home/<user>/.var/app/com.visualstudio.code/config/Code/User/globalStorage/ms-vscode-remote.remote-containers/data/docker-compose/docker-compose.devcontainer.build-1729258506835.yml build
[2024-10-18T13:35:07.126Z] Error: Command failed: /home/<user>/.local/bin/podman-host compose --project-name <project name> -f /home/<user>/<project>/docker-compose.yml -f /home/<user>/.var/app/com.visualstudio.code/config/Code/User/globalStorage/ms-vscode-remote.remote-containers/data/docker-compose/docker-compose.devcontainer.build-1729258506835.yml build
[2024-10-18T13:35:07.126Z]     at Pm (/home/<user>/.var/app/com.visualstudio.code/data/vscode/extensions/ms-vscode-remote.remote-containers-0.388.0/dist/spec-node/devContainersSpecCLI.js:430:525)
[2024-10-18T13:35:07.126Z]     at async ftA (/home/<user>/.var/app/com.visualstudio.code/data/vscode/extensions/ms-vscode-remote.remote-containers-0.388.0/dist/spec-node/devContainersSpecCLI.js:430:2476)
[2024-10-18T13:35:07.126Z]     at async htA (/home/<user>/.var/app/com.visualstudio.code/data/vscode/extensions/ms-vscode-remote.remote-containers-0.388.0/dist/spec-node/devContainersSpecCLI.js:410:3496)
[2024-10-18T13:35:07.126Z]     at async TtA (/home/<user>/.var/app/com.visualstudio.code/data/vscode/extensions/ms-vscode-remote.remote-containers-0.388.0/dist/spec-node/devContainersSpecCLI.js:482:4021)
[2024-10-18T13:35:07.126Z]     at async iB (/home/<user>/.var/app/com.visualstudio.code/data/vscode/extensions/ms-vscode-remote.remote-containers-0.388.0/dist/spec-node/devContainersSpecCLI.js:482:4963)
[2024-10-18T13:35:07.126Z]     at async wrA (/home/<user>/.var/app/com.visualstudio.code/data/vscode/extensions/ms-vscode-remote.remote-containers-0.388.0/dist/spec-node/devContainersSpecCLI.js:663:203)
[2024-10-18T13:35:07.126Z]     at async DrA (/home/<user>/.var/app/com.visualstudio.code/data/vscode/extensions/ms-vscode-remote.remote-containers-0.388.0/dist/spec-node/devContainersSpecCLI.js:662:14830)
[2024-10-18T13:35:07.127Z]     at async /home/<user>/.var/app/com.visualstudio.code/data/vscode/extensions/ms-vscode-remote.remote-containers-0.388.0/dist/spec-node/devContainersSpecCLI.js:482:1190
[2024-10-18T13:35:07.143Z] Stop (1298 ms): Run: /app/extra/vscode/code /home/<user>/.var/app/com.visualstudio.code/data/vscode/extensions/ms-vscode-remote.remote-containers-0.388.0/dist/spec-node/devContainersSpecCLI.js up --user-data-folder /home/<user>/.var/app/com.visualstudio.code/config/Code/User/globalStorage/ms-vscode-remote.remote-containers/data --docker-path /home/<user>/.local/bin/podman-host --container-session-data-folder /tmp/devcontainers-1248c3b6-2a23-4114-a0b8-e99fac641c2c1729258504414 --workspace-folder /home<user>/<project> --workspace-mount-consistency cached --gpu-availability detect --id-label devcontainer.local_folder=/home/<user>/<project> --id-label devcontainer.config_file=/home/<user>/<project>/.devcontainer/devcontainer.json --log-level debug --log-format json --config /home/<user>/<project>/.devcontainer/devcontainer.json --default-user-env-probe loginInteractiveShell --mount type=volume,source=vscode,target=/vscode,external=true --skip-post-create --update-remote-user-uid-default on --mount-workspace-git-root --include-configuration --include-merged-configuration
[2024-10-18T13:35:07.143Z] Exit code 1
[2024-10-18T13:35:07.151Z] Command failed: /app/extra/vscode/code /home/<user>/.var/app/com.visualstudio.code/data/vscode/extensions/ms-vscode-remote.remote-containers-0.388.0/dist/spec-node/devContainersSpecCLI.js up --user-data-folder /home/<user>/.var/app/com.visualstudio.code/config/Code/User/globalStorage/ms-vscode-remote.remote-containers/data --docker-path /home/<user>/.local/bin/podman-host --container-session-data-folder /tmp/devcontainers-1248c3b6-2a23-4114-a0b8-e99fac641c2c1729258504414 --workspace-folder /home/<user>/<project> --workspace-mount-consistency cached --gpu-availability detect --id-label devcontainer.local_folder=/home/<user>/<project> --id-label devcontainer.config_file=/home/<user>/<project>/.devcontainer/devcontainer.json --log-level debug --log-format json --config /home/<user>/<project>/.devcontainer/devcontainer.json --default-user-env-probe loginInteractiveShell --mount type=volume,source=vscode,target=/vscode,external=true --skip-post-create --update-remote-user-uid-default on --mount-workspace-git-root --include-configuration --include-merged-configuration
[2024-10-18T13:35:07.151Z] Exit code 1

OSError: Dockerfile not found in /home/<user>/<project> seems weird to me, because the Dockerfile is there. Outside of VSCode and Flatpak and what not I can build this Docker (well, Podman really) setup just fine, so I'm guessing the Flatpak sandbox is in the way somehow?