Move to xenial from trusty (#69)

Dockerfile

@@ -1,26 +1,24 @@
 # see https://hub.docker.com/_/ubuntu/ for versions, should be the same as on Travis for NodeMCU CI
-# 14.04 == trusty
-FROM ubuntu:14.04
-MAINTAINER marcelstoer
+# 16.04 == xenial
+FROM ubuntu:16.04
+LABEL maintainer="marcelstoer"
 
 # If you want to tinker with this Dockerfile on your machine do as follows:
 # - git clone https://github.com/marcelstoer/docker-nodemcu-build
-# - vim docker-nodemcu-build/Dockerfile
-# - docker build -t docker-nodemcu-build docker-nodemcu-build
+# - cd docker-nodemcu-build
+# - vim Dockerfile
+# - docker build -t docker-nodemcu-build .
 # - cd <nodemcu-firmware>
 # - docker run --rm -ti -v `pwd`:/opt/nodemcu-firmware docker-nodemcu-build build
 
+# Lint the final file with https://hadolint.github.io/hadolint/
+
 RUN apt-get update && apt-get install -y wget unzip git make python-serial srecord bc xz-utils gcc ccache tzdata
 
-# additionally required for ESP32 builds
-RUN apt-get update && apt-get install -y gperf python-pip python-dev flex bison build-essential libssl-dev libffi-dev libncurses5-dev libncursesw5-dev
-RUN pip install --upgrade pip &&\
-    pip install urllib3[secure] &&\
-    pip install --upgrade  setuptools &&\
-    pip install cryptography
-# Ubuntu trusty ships with make 3.8.x but ESP32 requires >= 4.0
-RUN wget https://mirrors.edge.kernel.org/ubuntu/pool/main/m/make-dfsg/make_4.1-6_amd64.deb &&\
-    sudo dpkg -i make_4.1-6_amd64.deb
+# additionally required for ESP32 builds as per https://nodemcu.readthedocs.io/en/dev-esp32/build/#ubuntu
+RUN apt-get install -y gperf python-pip python-dev flex bison build-essential libssl-dev libffi-dev libncurses5-dev libncursesw5-dev libreadline-dev
+
+RUN pip install --upgrade pip
 
 # Release some space...
 RUN rm -rf /var/lib/apt/lists/*
@@ -39,4 +37,4 @@ COPY build-esp8266 /opt/
 COPY configure-esp32 /opt/
 COPY lfs-image /opt/
 
-CMD /opt/cmd.sh
+CMD ["/opt/cmd.sh"]