Environment for ESP-IDF CI builds

Base on ubuntu:18.04 and setup ESP-IDF v3.2 with toolchain linux64-1.22.0-80-g6c4433a-5.2.0

docker run -it --rm larryli/esp-idf-ci-env
cd $IDF_PATH/examples/get-started/hello_world
make defconfig
make all V=1

You can write a .gitlab-ci.yml file like:

image: larryli/esp-idf-ci-env

stages:
  - build

# https://github.com/espressif/esp-idf-template
build_app:
  stage: build
  script:
    - make defconfig
    - make all V=1
  artifacts:
    paths:
      - build/bootloader/bootloader.bin
      - build/partitions_singleapp.bin
      - build/app-template.bin