Add a buildbot multibuild dir for building an armhf appimage

.buildbot/appimage/Dockerfile

@@ -0,0 +1,22 @@
+FROM ubuntu:bionic
+
+RUN apt-get update
+
+RUN apt-get install -yq --no-install-suggests --no-install-recommends \
+    ca-certificates software-properties-common \
+    build-essential libcap-dev libssl-dev \
+    python-all-dev python-setuptools wget \
+    git gtk-update-icon-cache \
+    binutils-multiarch crossbuild-essential-armhf
+
+RUN dpkg --add-architecture armhf
+
+RUN sed -iE "s|deb |deb [arch=amd64] |g" /etc/apt/sources.list \
+    && echo "deb [arch=armhf] http://ports.ubuntu.com/ubuntu-ports/ bionic main universe" >> /etc/apt/sources.list \
+    && echo "deb [arch=armhf] http://ports.ubuntu.com/ubuntu-ports/ bionic-updates main universe" >> /etc/apt/sources.list
+
+RUN apt-get update | true
+
+RUN apt-get install -yq libssl-dev:armhf
+
+ADD . .

.buildbot/appimage/build.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+wget -O appimage-builder-x86_64.AppImage \
+    https://github.com/AppImageCrafters/appimage-builder/releases/download/v1.1.0/appimage-builder-1.1.0-x86_64.AppImage \
+    && chmod +x appimage-builder-x86_64.AppImage
+
+APPIMAGE_EXTRACT_AND_RUN=1 ./appimage-builder-x86_64.AppImage \
+			--recipe packages/AppImage/AppImageBuilder.yml
+
+mkdir -p ../out
+mv PyBitmessage*.AppImage ../out