Change dockerfile to ubuntu 20.04 and correct OS check issue

docker/Dockerfile

@@ -1,11 +1,12 @@
-FROM ubuntu:latest
+FROM ubuntu:20.04
 
 RUN apt-get update && apt-get install -y git wget python3 python3-distutils sudo nano && sudo ln -s /usr/bin/python3 /usr/bin/python
 
 WORKDIR /app
 
 RUN git clone https://github.com/aman-goel/avr.git
 WORKDIR /app/avr
+RUN sed -i 's/-n "$(uname -a | grep Ubuntu)"/1 -eq 1/g' build.sh ## to get rid of OS check
 RUN chmod +x build.sh && chmod +x deps/build_deps.sh ## run build.sh manually, as it requires some y-s
 
 CMD ["bash"]