feat: add self-built Docker image from local hapi source

Add a new Docker image variant (tag: self) that uses compiled binaries
from the hapi self branch instead of installing from npm.

- self/Dockerfile: runtime image copying pre-built binary by arch
- self/build.sh: local build script from ../hapi/dist-exe
- .github/workflows/docker-build-self.yml: CI workflow to build and
  push multi-arch image to GHCR with tag "self"

via [HAPI](https://hapi.run)

Co-Authored-By: HAPI <[email protected]>

self/build.sh

@@ -0,0 +1,40 @@
+#!/bin/bash
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+HAPI_DIR="$(cd "$SCRIPT_DIR/../../hapi" && pwd)"
+
+ARCH=$(uname -m)
+case "$ARCH" in
+    x86_64)
+        HAPI_ARCH="amd64"
+        BINARY_DIR="bun-linux-x64-baseline"
+        ;;
+    aarch64|arm64)
+        HAPI_ARCH="arm64"
+        BINARY_DIR="bun-linux-arm64"
+        ;;
+    *)
+        echo "Unsupported architecture: $ARCH"
+        exit 1
+        ;;
+esac
+
+BINARY_PATH="$HAPI_DIR/cli/dist-exe/$BINARY_DIR/hapi"
+if [ ! -f "$BINARY_PATH" ]; then
+    echo "Binary not found: $BINARY_PATH"
+    echo "Please build hapi first: cd ../hapi && bun run build:single-exe"
+    exit 1
+fi
+
+cp "$BINARY_PATH" "$SCRIPT_DIR/hapi-${HAPI_ARCH}"
+
+docker build \
+    --build-arg "TARGETARCH=${HAPI_ARCH}" \
+    -t "hapi:self" \
+    -f "$SCRIPT_DIR/Dockerfile" \
+    "$SCRIPT_DIR/"
+
+rm -f "$SCRIPT_DIR/hapi-${HAPI_ARCH}"
+
+echo "Built hapi:self for ${HAPI_ARCH}"