build: add CircleCI

The big advantages are:
- everyone can download the build logs and the ipks
- we use our own docker image
- people with commit access can ssh into the build env

The disadvantages:
- need to push new commits to restart the build

I haven't reimplemented the commit message checks as this should be replaced
with a separate script doing only that so we can require it
(https://help.github.com/articles/enabling-required-status-checks/)

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

.circleci/Dockerfile

@@ -0,0 +1,25 @@
+FROM debian:9
+
+RUN apt update && apt install -y \
+build-essential \
+jq \
+gawk \
+gettext \
+git \
+libncurses5-dev \
+libssl-dev \
+subversion \
+zlib1g-dev \
+&& rm -rf /var/lib/apt/lists/*
+
+# LEDE Build System (LEDE GnuPG key for unattended build jobs)
+RUN curl 'https://git.openwrt.org/?p=keyring.git;a=blob_plain;f=gpg/626471F1.asc' | gpg --import \
+ && echo '54CC74307A2C6DC9CE618269CD84BCED626471F1:6:' | gpg --import-ownertrust
+
+# LEDE Release Builder (17.01 "Reboot" Signing Key)
+RUN curl 'https://git.openwrt.org/?p=keyring.git;a=blob_plain;f=gpg/D52BBB6B.asc' | gpg --import \
+ && echo 'B09BE781AE8A0CD4702FDCD3833C6010D52BBB6B:6:' | gpg --import-ownertrust
+
+# OpenWrt Release Builder (18.06 Signing Key)
+RUN curl 'https://git.openwrt.org/?p=keyring.git;a=blob_plain;f=gpg/17E1CE16.asc' | gpg --import \
+ && echo '6768C55E79B032D77A28DA5F0F20257417E1CE16:6:' | gpg --import-ownertrust