Add Docker build args for NEXT_PUBLIC env vars

Next.js NEXT_PUBLIC_* variables are baked in at build time, not
runtime. Without ARG declarations in the Dockerfile, Dokku's
--build-arg flags have no effect and PostHog/API URLs are undefined
in the client bundle.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -7,6 +7,15 @@ COPY package*.json ./
 RUN npm ci
 
 COPY . .
+
+# Next.js NEXT_PUBLIC_* vars must be present at build time
+ARG NEXT_PUBLIC_API_URL
+ARG NEXT_PUBLIC_POSTHOG_KEY
+ARG NEXT_PUBLIC_POSTHOG_HOST
+ENV NEXT_PUBLIC_API_URL=$NEXT_PUBLIC_API_URL
+ENV NEXT_PUBLIC_POSTHOG_KEY=$NEXT_PUBLIC_POSTHOG_KEY
+ENV NEXT_PUBLIC_POSTHOG_HOST=$NEXT_PUBLIC_POSTHOG_HOST
+
 RUN npm run build
 
 # Production stage