fix(docker): target the dev stage when building for local development

docker-compose.override.yml only overrode the dockerfile, so it kept
inheriting target: final from docker-compose.yml — but Dockerfile.dev
is single-stage and has no final stage, so the dev build failed. Name
the stage in Dockerfile.dev and target it explicitly from the
override.

Dockerfile.dev

@@ -1,4 +1,4 @@
-FROM dunglas/frankenphp:1-php8.4-alpine
+FROM dunglas/frankenphp:1-php8.4-alpine AS dev
 
 RUN apk add --no-cache icu-dev libzip-dev \
     && docker-php-ext-install -j$(nproc) intl opcache zip \