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/Makefile

@@ -0,0 +1,19 @@
+# Ten31Portal StartOS package build
+# Docker image must be built separately before packing:
+#   docker build -f deploy/Dockerfile -t ten31portal:local .  (from repo root)
+#   cd deploy && make x86
+
+LICENSE := ../LICENSE
+
+include s9pk.mk
+
+# Override the license path for s9pk pack
+$(BASE_NAME).s9pk: $(INGREDIENTS) .git/HEAD .git/index
+	@$(MAKE) --no-print-directory ingredients
+	@echo "   Packing '$@'..."
+	start-cli s9pk pack --license $(LICENSE) -o $@
+
+$(BASE_NAME)_%.s9pk: $(INGREDIENTS) .git/HEAD .git/index
+	@$(MAKE) --no-print-directory ingredients
+	@echo "   Packing '$@'..."
+	start-cli s9pk pack --arch=$* --license $(LICENSE) -o $@