Enable docker for GPU builds

I finally got nvidia-docker2 installed and actually working on my Gentoo
machine (I had to disable their restriction of using GPU devices to
users in the 'video' group, since the nvidia container runtime hook
didn't run as a user with those privileges, and I can't figure out what
user it was running as).

The other GPU-enabled node I added to the Jenkins array runs Ubuntu, and
that one already has nvidia-docker2 installed and running correctly on
it.

devtools/Jenkinsfile

@@ -5,7 +5,12 @@ pipeline {
         stage("Build and test") {
             parallel {
                 stage("Build and test CUDA platform") {
-                    agent { label "cuda" }
+                    agent {
+                        dockerfile {
+                            dir "devtools/ci/jenkins"
+                            label "cuda"
+                        }
+                    }
                     steps {
                         sh '''#!/bin/bash -lex
                             git clean -fxd && git checkout .
@@ -19,7 +24,12 @@ pipeline {
                 }
 
                 stage("Build and test OpenCL platform") {
-                    agent { label "cuda" }
+                    agent {
+                        dockerfile {
+                            dir "devtools/ci/jenkins"
+                            label "cuda"
+                        }
+                    }
                     steps {
                         sh '''#!/bin/bash -lex
                             git clean -fxd && git checkout .
@@ -36,7 +46,6 @@ pipeline {
                     agent {
                         dockerfile {
                             dir "devtools/ci/jenkins"
-                            label "docker"
                         }
                     }
 

devtools/ci/jenkins/Dockerfile

-FROM ubuntu:xenial
+FROM nvidia/cuda:10.0-devel-ubuntu18.04
 
 ENV PATH="/opt/miniconda/bin:${PATH}"