fix(admin): keep admin app in the docker build context

The root .dockerignore excluded apps/admin, so the monorepo build context
dropped that workspace entirely: npm ci skipped it and
`npm run build -w @sauci/admin` failed with "No workspaces found".
The ignore file was authored for the mcp image and later extended with
re-includes for web; admin never got the same treatment.

Re-include apps/admin, and re-exclude node_modules/dist last so the
re-includes cannot pull host build artefacts into any image context.

Verified with `docker build -f apps/admin/Dockerfile .`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.dockerignore

@@ -39,3 +39,11 @@ terraform
 .claude
 .codex
 .ralph
+
+# The admin app builds from this same monorepo context (apps/admin/Dockerfile).
+!apps/admin
+!apps/admin/**
+
+# Host build artefacts must never leak into any image context.
+**/node_modules
+**/dist