feat(server): add 11.0.0-rc1 image from GitHub release tarball

[DeepDiver1975]

Summary

Replaces the 11.0.0-prealpha image (rolling daily-master) with 11.0.0-rc1, built from the pinned GitHub release tarball instead of the download server:

• Tarball: https://github.com/owncloud/core/releases/download/v11.0.0-rc1/owncloud-complete-20260629.tar.bz2
• SHA256: a70b060ab30d93ae4c21eea49051684db0cc5c293b70d444cc4591f9c620fb43

Same base v24.04 / PHP 8.3. Published as a version-only tag (11.0.0-rc1, no latest/floating tags) since it's a pre-release.

Changes

• .github/workflows/main.yml — swap the prealpha matrix entry for 11.0.0-rc1 (GitHub release tarball, no extra-tags).
• v24.04/11.0.0-prealpha/.trivyignore → renamed to v24.04/11.0.0-rc1/.trivyignore (content unchanged — the Windows-only CVE is base/codeline-shared).
• README.md — update Docker Tags line and the "Running occ commands" reference.
• CHANGELOG.md — new 2026-06-29 entry.
• .drone.star (legacy CI) intentionally left untouched.

Verification

• Workflow YAML parses; rc1 entry correct.
• Local docker build succeeded — tarball fetched from the GitHub release, unpacked into /var/www/owncloud, image built clean. version.php reports $OC_VersionString = '11.0.0-rc1'.

Note: the reusable docker-build.yml workflow only accepts TARBALL_URL (no sha256 input), so GitHub Actions builds rc1 without hash verification — consistent with the existing 10.16.3 entry. The verified SHA256 is recorded above for reference.

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[DeepDiver1975]

Looks good — sound and scoped. Comment-only (I authored this, so I can't self-approve).

Verified

• Tarball https://github.com/owncloud/core/releases/download/v11.0.0-rc1/owncloud-complete-20260629.tar.bz2 resolves (200, ~130MB) and is a real asset on the v11.0.0-rc1 release (isPrerelease: true). Correct version/source.
• Tag wiring is correct for an RC: the matrix entry has no extra-tags, so latest/10/11 are NOT moved onto the pre-release. Only the exact 11.0.0-rc1 tag is produced — exactly right for a release candidate.
• 1:1 swap of the old 11.0.0-prealpha (daily master) entry; README + CHANGELOG + trivyignore-dir rename all consistent. No secrets, no stray changes.

One non-blocking point

• v24.04/Dockerfile.multiarch consumes the tarball via ADD --unpack ${TARBALL_URL} with no integrity check, even though the release publishes .sha256/.asc sidecars. This is pre-existing (prealpha did the same), so not a regression — but switching to an immutable GitHub release asset is a natural moment to pin the SHA256. Worth a follow-up, not a blocker here.

Holding for CI — the two build jobs were still in progress at review time.

[DeepDiver1975]

(Review attribution for the comment above.)

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