fix(release): inject buildinfo into Docker image binaries

[upsetbit]

Problem

Binaries inside the published Docker images report version="dev (none, unknown)" at startup and on --version — observed on the production prosa-server/prosa-panel containers running the v0.21.2 images. The root Dockerfile compiles with bare -s -w, without the internal/buildinfo ldflags that GoReleaser injects into the tarball binaries.

Fix

• Dockerfile: the shared build stage takes VERSION / COMMIT / BUILD_DATE build args (defaulting to the existing dev / none / unknown) and wires them to the same -X github.com/c3-oss/prosa/internal/buildinfo.* ldflags GoReleaser uses.
• release.yml: a "Compute Docker build metadata" step mirrors GoReleaser's {{.Version}} / {{.Commit}} / {{.Date}} (tag without the v prefix, commit SHA, UTC RFC 3339 date) and all three build-push-action steps pass the args — identical args across the three targets, so the buildx cache for the build stage is still shared.
• docs/distribution/docker.md: the embedded Dockerfile excerpt and notes follow the change.

Validation

Local build with args produces the expected output:

$ docker build --target prosa --build-arg VERSION=0.0.0-test --build-arg COMMIT=abc1234 --build-arg BUILD_DATE=2026-06-12T00:00:00Z -t t .
$ docker run --rm t --version
prosa version 0.0.0-test (abc1234, 2026-06-12T00:00:00Z)

just lint-md and just quality green. Takes effect on the next tagged release.

🤖 Generated with Claude Code

---

^{Need help on this PR? Tag /codesmith with what you need. Autofix is disabled.}