CircleCI: Add support for usign signatures

It appears snapshot target builds have switched from GPG signatures
(sha256sums.asc) to usign signatures (sha256sums.sig).

This adds support for verifying these usign signatures. (GPG signatures
will also be verified if found.)

This also restores the alphabetical ordering of packages to be installed
by apt-get.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

.circleci/Dockerfile

@@ -5,9 +5,11 @@ FROM debian:9
 # v1.0   - Initial version by Etienne Champetier
 # v1.0.1 - Run as non-root, add unzip, xz-utils
 # v1.0.2 - Add bzr
+# v1.0.3 - Verify usign signatures
 
 RUN apt update && apt install -y \
 build-essential \
+bzr \
 curl \
 jq \
 gawk \
@@ -16,13 +18,13 @@ git \
 libncurses5-dev \
 libssl-dev \
 python \
+signify-openbsd \
 subversion \
-bzr \
 time \
-wget \
-zlib1g-dev \
 unzip \
+wget \
 xz-utils \
+zlib1g-dev \
 && rm -rf /var/lib/apt/lists/*
 
 RUN useradd -c "OpenWrt Builder" -m -d /home/build -s /bin/bash build
@@ -40,3 +42,7 @@ RUN curl 'https://git.openwrt.org/?p=keyring.git;a=blob_plain;f=gpg/D52BBB6B.asc
 # 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
+
+# LEDE Build System (LEDE usign key for unattended build jobs)
+RUN curl 'https://git.openwrt.org/?p=keyring.git;a=blob_plain;f=usign/b5043e70f9a75cde' --create-dirs \
+ -o /home/build/usign/b5043e70f9a75cde