Variables not expanded in platform option of Docker image

Summary

I was very happy to see support for expanding variables in the image:docker:platform key with #38290 (closed) and now that 17.8 was released and I installed/restarted my gitlab-runners I had height hopes that it works, but unfortunately not. I still get this error from the runner. See below for the job log.

Steps to reproduce

.gitlab-ci.yml

toolchain:
  stage: Docker
  needs: []
  parallel:
    matrix:
      - IMAGE: [gcc, clang]
        PLATFORM: ['amd64', 'arm64', 'ppc64le', 'riscv64']
      - IMAGE: [nvhpc]
        PLATFORM: ['amd64', 'arm64']
      - IMAGE: [oneapi, rocm]
        PLATFORM: ['amd64']
  image:
    name: …${IMAGE}…
    docker:
      platform: linux/${PLATFORM}

Actual behavior

gitlab-runner does not expand ${PLATFORM} and pulls with an invalid platform name.

Expected behavior

gitlab-runner expands ${PLATFORM} and pulls a valid platform image.

Relevant logs and/or screenshots

job log

Running with gitlab-runner 17.8.0 (e4f782b3)
  on gl-com-wesarg t3_yggVtp, system ID: s_c91cd49d5ce7
Preparing the "docker" executor 00:04
Using Docker executor with image … ...
Using helper image:  registry.gitlab.com/gitlab-org/gitlab-runner/gitlab-runner-helper:x86_64-v17.8.0
Authenticating with credentials from job payload (GitLab Registry)
Pulling docker image registry.gitlab.com/gitlab-org/gitlab-runner/gitlab-runner-helper:x86_64-v17.8.0 ...
Using docker image sha256:2e035acdb749b559dbdb37734c6673d8aeea577268726f4fcad761055e844264 for registry.gitlab.com/gitlab-org/gitlab-runner/gitlab-runner-helper:x86_64-v17.8.0 with digest registry.gitlab.com/gitlab-org/gitlab-runner/gitlab-runner-helper@sha256:7e2d40f0046ad465127b5652650933b517cc53f5d5c4dbdcc3940cc1594aba43 ...
Pulling docker image … for platform linux/${PLATFORM} ...
WARNING: Failed to pull image with policy "always": Error response from daemon: "${platform}" is an invalid component of "linux/${platform}": platform specifier component must match "^[A-Za-z0-9_-]+$": invalid argument (manager.go:254:0s)
ERROR: Job failed: failed to pull image "…" with specified policies [always]: Error response from daemon: "${platform}" is an invalid component of "linux/${platform}": platform specifier component must match "^[A-Za-z0-9_-]+$": invalid argument (manager.go:254:0s)

Environment description

https://gitlab.com/bertwesarg/scorep/-/jobs/8881845945

config.toml contents

[[runners]]
  name = "gl-com-wesarg"
  url = "https://gitlab.com"
  id = 45281846
  token = "…"
  token_obtained_at = 2025-01-14T14:08:27Z
  token_expires_at = 0001-01-01T00:00:00Z
  executor = "docker"
  [runners.custom_build_dir]
  [runners.cache]
    MaxUploadedArchiveSize = 0
    [runners.cache.s3]
    [runners.cache.gcs]
    [runners.cache.azure]
  [runners.docker]
    tls_verify = false
    image = "ubunt:24.04"
    privileged = false
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/cache"]
    shm_size = 0
    network_mtu = 0

Used GitLab Runner version

$ gitlab-runner --version
Version:      17.8.0
Git revision: e4f782b3
Git branch:   17-8-stable
GO version:   go1.23.2 X:cacheprog
Built:        unknown
OS/Arch:      linux/amd64