Set HCAPTCHA_SITE_KEY as a build environment variable

Added NEXT_PUBLIC_HCAPTCHA_SITE_KEY to the Dockerfile to pass the hCaptcha site key as an environment variable during the build process. This ensures the key is appropriately injected into the Next.js build.

frontend/Dockerfile

@@ -13,6 +13,9 @@ RUN npm ci
 # Copy entire project
 COPY . .
 
+# Set it as an environment variable for the build process
+ENV NEXT_PUBLIC_HCAPTCHA_SITE_KEY=${HCAPTCHA_SITE_KEY}
+
 # Build the Next.js app
 RUN npm run build