ci: use the docker-compose Dependabot ecosystem for the devcontainer

[maia-andre]

Resolves: #8121

📝 Summary

The Dependabot entry added in #8033 for /.devcontainer uses the docker ecosystem, which only reads Dockerfiles and Kubernetes manifests. The devcontainer declares its images in docker-compose.yml, so every daily job failed with No Dockerfiles nor Kubernetes YAML found in /.devcontainer (e.g. https://github.com/LibreSign/libresign/actions/runs/33188239975) and the ghcr.io/librecodecoop/nextcloud-dev:8.3 tag was never checked.

This switches the entry to the dedicated docker-compose ecosystem. Nothing else changes.

Side effect to be aware of: Dependabot will now also see mariadb:10.6 in the same file and may propose MariaDB updates. If major bumps are not wanted there, an ignore rule for mariadb with update-types: ["version-update:semver-major"] can be added — I left it out to keep this PR to the fix, happy to add it if preferred. axllent/mailpit, redis and nextcloud-dev-nginx:latest carry no version tag and are skipped.

🧪 How to test

Dependabot version updates only run from the default branch, so this cannot be exercised from a fork PR. After merge, the next scheduled job should show up as docker-compose in /.devcontainer (instead of docker in /.devcontainer) under Insights → Dependency graph → Dependabot, without the "No Dockerfiles" error, and open a PR whenever a newer nextcloud-dev tag exists.

✅ Checklist

• I have read and followed the contribution guide.

🤖 AI (if applicable)

• The content of this PR was partially or fully generated using AI

[vitormattos]

/backport to stable35

[vitormattos]

/backport to stable34

[vitormattos]

/backport to stable33

[vitormattos]

/backport to stable32

[backportbot-libresign]

The backport to stable32 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable32
git pull origin stable32

# Create the new backport branch
git checkout -b backport/8123/stable32

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 2bfe6288

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/8123/stable32

Error: Failed to check for changes with origin/stable32: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[backportbot-libresign]

The backport to stable33 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable33
git pull origin stable33

# Create the new backport branch
git checkout -b backport/8123/stable33

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 2bfe6288

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/8123/stable33

Error: Failed to check for changes with origin/stable33: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[backportbot-libresign]

The backport to stable34 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable34
git pull origin stable34

# Create the new backport branch
git checkout -b backport/8123/stable34

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 2bfe6288

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/8123/stable34

Error: Failed to check for changes with origin/stable34: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[backportbot-libresign]

The backport to stable35 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable35
git pull origin stable35

# Create the new backport branch
git checkout -b backport/8123/stable35

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 2bfe6288

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/8123/stable35

Error: Failed to push branch backport/8123/stable35: remote: Invalid username or token. Password authentication is not supported for Git operations.
fatal: Authentication failed for 'https://github.com/LibreSign/libresign.git/'

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.