diff --git a/Dockerfile.standalone b/Dockerfile.standalone
index 2c314fa..68c2be0 100644
--- a/Dockerfile.standalone
+++ b/Dockerfile.standalone
@@ -77,18 +77,10 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 RUN curl -fsSL https://raw.githubusercontent.com/arduino/arduino-cli/master/install.sh \
     | BINDIR=/usr/local/bin sh
 
-# Initialize arduino-cli config, add board manager URLs, then install cores
-# - RP2040: earlephilhower fork for Raspberry Pi Pico
-# - ESP32:  Espressif official (covers ESP32, ESP32-S3, ESP32-C3, etc.)
-RUN arduino-cli config init \
-    && arduino-cli config add board_manager.additional_urls \
-       https://github.com/earlephilhower/arduino-pico/releases/download/global/package_rp2040_index.json \
-    && arduino-cli config add board_manager.additional_urls \
-       https://raw.githubusercontent.com/espressif/arduino-esp32/gh-pages/package_esp32_index.json \
-    && arduino-cli core update-index \
-    && arduino-cli core install arduino:avr \
-    && arduino-cli core install rp2040:rp2040 \
-    && arduino-cli core install esp32:esp32
+# Only install arduino-cli binary here. Core installation (arduino:avr,
+# rp2040:rp2040, esp32:esp32) is done at first boot by entrypoint.sh
+# and persisted in the mounted /root/.arduino15 volume.
+# This keeps the image small (~500MB vs ~2.5GB) and builds fast.
 
 WORKDIR /app