[BUILD] Dockerfile and Jenkinsfile revisited (#2514)

Includes:
  - Dockerfile changes
    - Dockerfile clean up
    - Fix execution privileges of files used from Dockerfile.
    - New Dockerfile entrypoint to replace with_user script
    - Defined a placeholders for CPU testing (script and Dockerfile)
  - Jenkinsfile
    - Jenkins file milestone defined
    - Single source code checkout and propagation via stash/unstash
    - Bash needs to be explicitly used in launching make build, since we need
access to environment
    - Jenkinsfile build factory for cmake and make style of jobs
    - Archivation of artifacts (*.so, *.whl, *.egg) produced by cmake build

Missing:
  - CPU testing
  - Python3 env build and testing

tests/ci_build/test_gpu.sh

@@ -1,10 +1,5 @@
 #!/usr/bin/env bash
 
-mkdir build
-cd build
-cmake .. -DPLUGIN_UPDATER_GPU=ON
-make
-cd ..
 cd python-package
 python setup.py install --user
 cd ../plugin/updater_gpu