Layer 1 of the runtime-rollout plan. Decouples publish from promotion by
giving operators a `runtime_image_pins` table the provisioner consults at
container-create time. No row = legacy `:latest` behavior; row present =
provisioner pulls `<base>@sha256:<digest>`. One bad publish no longer
breaks every workspace simultaneously.
Mechanics:
- Migration 047: `runtime_image_pins` (template_name PK + sha256 digest +
audit columns) and `workspaces.runtime_image_digest` (nullable, with
partial index) for "show me workspaces still on the old digest" queries.
- `resolveRuntimeImage` (handlers/runtime_image_pin.go): looks up the
pin, returns `<base>@sha256:<digest>` on hit, "" on miss/error so the
provisioner falls through to the legacy tag map. Availability over
pinning — any DB error logs and returns "" rather than blocking the
provision. `WORKSPACE_IMAGE_LOCAL_OVERRIDE=1` short-circuits the
lookup so devs rebuilding template images locally see their fresh
build.
- `WorkspaceConfig.Image` carries the resolved value into the
provisioner. `selectImage` honors it ahead of the runtime→tag map and
falls back to DefaultImage on unknown runtime.
- The existing `imageTagIsMoving` predicate (#215) already returns false
on `@sha256:` form, so digest pins skip the force-pull path naturally.
Tests:
- Handler-side (sqlmock): no-pin/db-error/with-pin/empty/unknown/local-
override paths cover every branch of `resolveRuntimeImage`.
- Provisioner-side: `selectImage` table covers explicit-image preference,
runtime-map fallback, unknown-runtime → default, empty-config →
default. Plus a struct-literal compile-time pin on `Image` so a future
refactor can't silently drop the field.
Layer 2 (per-ring routing via `workspaces.runtime_image_digest`) and the
admin promote/rollback endpoint ride on top of this and ship separately.
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