Improve atmega Dockerfile apt handling

- Set DEBIAN_FRONTEND as an environment variable.
- Add --no-install-recommends to apt-get install.

firmware/atmega/Dockerfile

@@ -1,7 +1,9 @@
 FROM ubuntu:22.04
 
+ENV DEBIAN_FRONTEND=noninteractive
+
 RUN apt-get update && \
-    DEBIAN_FRONTEND=noninteractive apt-get install -y \
+    apt-get install -y --no-install-recommends\
     build-essential \
     gcc-avr \
     avrdude \