ci: run coverity from a nightly VSTS build

.vsts-nightly.yml

+resources:
+- repo: self
+
+phases:
+- phase: coverity
+  displayName: 'Coverity'
+  queue:
+    name: 'Hosted Linux Preview'
+  steps:
+  - task: Docker@0
+    displayName: Build
+    inputs:
+      action: 'Run an image'
+      imageName: 'libgit2/trusty-openssl:latest'
+      volumes: |
+       $(Build.SourcesDirectory):/src
+       $(Build.BinariesDirectory):/build
+      envVars: |
+       COVERITY_TOKEN=$(COVERITY_TOKEN)
+      workDir: '/build'
+      containerCommand: '/src/ci/coverity.sh'
+      detached: false