fix(compose): platform build context must be repo root, not ./platform

The platform Dockerfile COPYs paths relative to the repo root —
\`COPY platform/go.mod\`, \`COPY platform/migrations\`,
\`COPY workspace-configs-templates\`. The compose file was setting
\`context: ./platform\`, which silently caused those COPY layers to
miss + stop invalidating cache.

Symptom (caught 2026-04-16 10:22 UTC): after PR #417 (memory schema
migration 023) merged + I ran \`docker compose up -d --build platform\`,
the rebuild was a no-op. Image SHA didn't change, container booted with
old migration set, \`Applied 22 migrations\` instead of the expected 23.
Migration 023 file was on disk locally but never reached the image.

Workaround was \`docker build -t molecule-monorepo-platform:fresh -f
platform/Dockerfile .\` from repo root → SHA changed, migration 023
applied. This commit makes \`docker compose up -d --build platform\`
work correctly without the manual workaround.

CI workflow already builds with \`context: .\` + \`file: ./platform/Dockerfile\`
(per the comment at the top of platform/Dockerfile). This change just
aligns the local compose file with what CI does.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -102,8 +102,17 @@ services:
   # --- Platform ---
   platform:
     build:
-      context: ./platform
-      dockerfile: Dockerfile
+      # Build context MUST be repo root, not ./platform — the Dockerfile
+      # COPYs `platform/migrations`, `platform/go.mod`,
+      # `workspace-configs-templates/` etc. via repo-relative paths so it
+      # can bake in templates + migrations alongside the platform binary.
+      # When context was ./platform earlier, docker silently cached an
+      # earlier image (the COPY platform/migrations resolved to nothing
+      # under ./platform/, so layers stopped invalidating) — manifested
+      # as migration 023 not landing after PR #417 merged. CI workflow
+      # already uses context=. , this aligns local with CI.
+      context: .
+      dockerfile: platform/Dockerfile
     depends_on:
       postgres:
         condition: service_healthy