improve readability of Dockerfile

Switch to docker/dockerfile:1 syntax. This requires buildx.

Dockerfile

@@ -1,17 +1,25 @@
+# syntax=docker/dockerfile:1
 FROM ruby:3.2-alpine
 
 WORKDIR /app
 COPY Gemfile Gemfile.lock /app/
-RUN set -ex \
- && apk add --no-cache --virtual build-dependencies \
-      make \
-      gcc \
-      libc-dev \
-      git \
- && bundle config set --local deployment "true" \
- && bundle config set --local without "development test" \
- && bundle install \
- && apk del build-dependencies
+
+# hadolint ignore=DL3018
+RUN <<BASH
+  set -ex
+
+  apk add --no-cache --virtual build-dependencies \
+    make \
+    gcc \
+    libc-dev \
+    git
+
+  bundle config set --local deployment "true"
+  bundle config set --local without "development test"
+  bundle install
+
+  apk del build-dependencies
+BASH
 
 COPY . .