Add drone ci builds

Adds CI builds (using drone) for various ubuntu/debian builds on
amd64/arm64/i386/armhf.

.drone.jsonnet

@@ -0,0 +1,41 @@
+local debian_pipeline(name, image, arch='amd64', deps='g++ libsodium-dev libzmq3-dev', cmake_extra='', extra_cmds=[], allow_fail=false) = {
+    kind: 'pipeline',
+    type: 'docker',
+    name: name,
+    platform: { arch: arch },
+    steps: [
+        {
+            name: 'build',
+            image: image,
+            [if allow_fail then "failure"]: "ignore",
+            commands: [
+                'apt-get update',
+                'apt-get install -y eatmydata',
+                'eatmydata apt-get dist-upgrade -y',
+                'eatmydata apt-get install -y cmake git ninja-build pkg-config ccache ' + deps,
+                'git submodule update --init --recursive',
+                'mkdir build',
+                'cd build',
+                'cmake .. -G Ninja -DCMAKE_CXX_FLAGS=-fdiagnostics-color=always -DCMAKE_CXX_COMPILER_LAUNCHER=ccache ' + cmake_extra,
+                'ninja -v',
+                './tests/tests --use-colour yes'
+            ] + extra_cmds,
+        }
+    ]
+};
+
+[
+    debian_pipeline("Ubuntu focal (amd64)", "ubuntu:focal"),
+    debian_pipeline("Ubuntu bionic (amd64)", "ubuntu:bionic", deps='libsodium-dev g++-8',
+                    cmake_extra='-DCMAKE_C_COMPILER=gcc-8 -DCMAKE_CXX_COMPILER=g++-8'),
+    debian_pipeline("Debian sid (amd64)", "debian:sid"),
+    debian_pipeline("Debian sid/clang-10 (amd64)", "debian:sid", deps='clang-10 lld-10 libsodium-dev libzmq3-dev',
+                    cmake_extra='-DCMAKE_C_COMPILER=clang-10 -DCMAKE_CXX_COMPILER=clang++-10 ' + std.join(' ', [
+                        '-DCMAKE_'+type+'_LINKER_FLAGS=-fuse-ld=lld-10' for type in ['EXE','MODULE','SHARED','STATIC']])),
+    debian_pipeline("Debian buster (amd64)", "debian:buster"),
+    debian_pipeline("Debian buster (i386)", "i386/debian:buster"),
+    debian_pipeline("Ubuntu bionic (ARM64)", "ubuntu:bionic", arch="arm64", deps='libsodium-dev g++-8',
+                    cmake_extra='-DCMAKE_C_COMPILER=gcc-8 -DCMAKE_CXX_COMPILER=g++-8'),
+    debian_pipeline("Debian sid (ARM64)", "debian:sid", arch="arm64"),
+    debian_pipeline("Debian buster (armhf)", "arm32v7/debian:buster", arch="arm64"),
+]