CI: GitHub Actions build and release workflows

- build.yml: runs on push to main, builds Docker image + packs s9pk,
  uploads artifact
- release.yml: runs on version tags (v*.*), builds and creates GitHub
  release with s9pk attached
- Dockerfile: build context is repo root (docker build -f deploy/Dockerfile .)
- Manifest: switched to dockerTag (pre-built image pattern)
- Makefile + s9pk.mk: local build support

deploy/startos/manifest/index.ts

@@ -16,8 +16,8 @@ export const manifest = setupManifest({
   volumes: ['main'],
   images: {
     main: {
-      source: { dockerBuild: { dockerfile: './Dockerfile', workdir: '.' } },
-      arch: ['x86_64', 'aarch64'],
+      source: { dockerTag: 'ten31portal:local' },
+      arch: ['x86_64'],
     },
   },
   alerts: {