Add Docker support with Dockerfile, entrypoint script, and CI workflows

- Introduced a Dockerfile for building the application with Go and Node.js dependencies.
- Added a docker-entrypoint.sh script to manage container execution.
- Created a .dockerignore file to exclude unnecessary files from the Docker context.
- Updated Makefile to include a new target for building the Docker image.
- Added CI workflows for testing, formatting, and releasing artifacts in Gitea.

Makefile

@@ -3,8 +3,9 @@ CMD_DIR := ./cmd/scratchbox
 BIN_DIR := ./bin
 BIN := $(BIN_DIR)/$(APP_NAME)
 CONFIG ?= config.yaml
+DOCKER_IMAGE ?= scratchbox:dev
 
-.PHONY: help run dev config build test test-race fmt clean
+.PHONY: help run dev config build docker test test-race fmt clean
 
 help:
 	@echo "Targets:"
@@ -12,6 +13,7 @@ help:
 	@echo "  make dev    - Build and run server (CONFIG=$(CONFIG))"
 	@echo "  make config - Write default config to $(CONFIG) and generate sibling config.key if missing"
 	@echo "  make build  - Build Svelte UI and binary to $(BIN)"
+	@echo "  make docker - Build Docker image (DOCKER_IMAGE=$(DOCKER_IMAGE))"
 	@echo "  make test   - Run all Go tests with timeout and shuffle"
 	@echo "  make test-race - Run all Go tests with race detector and timeout"
 	@echo "  make fmt    - Format Go code"
@@ -32,7 +34,10 @@ config:
 build: clean
 	npm --prefix ./web/ui run build
 	mkdir -p $(BIN_DIR)
-	go build -o $(BIN) $(CMD_DIR)
+	CGO_ENABLED=0 go build -o $(BIN) $(CMD_DIR)
+
+docker:
+	docker build -t $(DOCKER_IMAGE) .
 
 test:
 	go test -timeout 30s -shuffle=on ./...