ci: introduce nightly arm docker builds

Use multiarch arm32 and arm64 docker images to run Xenial-based images
for those platforms.  We can support all the tests on ARM32 and 64
_except_ the proxy-based tests.  Our proxy on ARM seems regrettably
unstable, either due to some shoddy dependencies (with native code?)
or the JREs themselves.

Run these platforms as part of our nightly builds; do not run them
during pull request or CI validation.

azure-pipelines/docker.yml

 # These are the steps used in a container-based build in VSTS.
 steps:
+- ${{ if eq(parameters.qemu, 'true') }}:
+  - script: docker run --rm --privileged multiarch/qemu-user-static:register --reset
+    displayName: 'Register Docker QEMU'
+
 - task: docker@0
   displayName: Build
   inputs:

azure-pipelines/nightly.yml

@@ -2,6 +2,34 @@ resources:
 - repo: self
 
 jobs:
+- job: linux_arm32_xenial_gcc_openssl
+  displayName: 'Linux (arm32; Xenial; GCC; OpenSSL)'
+  pool:
+    vmImage: 'Ubuntu 16.04'
+  steps:
+  - template: docker.yml
+    parameters:
+      qemu: 'true'
+      imageName: 'libgit2/xenial-arm32:test'
+      environmentVariables: |
+       CC=gcc
+       CMAKE_OPTIONS=-DUSE_HTTPS=OpenSSL
+       SKIP_PROXY_TESTS=true
+
+- job: linux_arm64_xenial_gcc_openssl
+  displayName: 'Linux (arm64; Xenial; GCC; OpenSSL)'
+  pool:
+    vmImage: 'Ubuntu 16.04'
+  steps:
+  - template: docker.yml
+    parameters:
+      qemu: 'true'
+      imageName: 'libgit2/xenial-arm64:test'
+      environmentVariables: |
+       CC=gcc
+       CMAKE_OPTIONS=-DUSE_HTTPS=OpenSSL
+       SKIP_PROXY_TESTS=true
+
 - job: coverity
   displayName: 'Coverity'
   pool: