[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/entrypoint.sh

@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+
+# This script is a wrapper creating the same user inside container as the one
+# running the ci_build.sh outside the container. It also set the home directory
+# for the user inside container to match the same absolute path as the workspace
+# outside of container.  Do not run this manually. It does not make sense. It is
+# intended to be called by ci_build.sh only.
+
+set -e
+
+COMMAND=("$@")
+
+if ! touch /this_is_writable_file_system; then
+  echo "You can't write to your filesystem!"
+  echo "If you are in Docker you should check you do not have too many images" \
+      "with too many files in them. Docker has some issue with it."
+  exit 1
+else
+  rm /this_is_writable_file_system
+fi
+
+if [[ -n $CI_BUILD_UID ]] && [[ -n $CI_BUILD_GID ]]; then
+    groupadd -o -g "${CI_BUILD_GID}" "${CI_BUILD_GROUP}"
+    useradd -o -m -g "${CI_BUILD_GID}" -u "${CI_BUILD_UID}" \
+        "${CI_BUILD_USER}"
+    export HOME="/home/${CI_BUILD_USER}"
+    shopt -s dotglob
+    cp -r /root/* "$HOME/"
+    chown -R "${CI_BUILD_UID}:${CI_BUILD_GID}" "$HOME"
+
+    # Allows project-specific customization
+    if [[ -e "/workspace/.pre_entry.sh" ]]; then
+        gosu "${CI_BUILD_UID}:${CI_BUILD_GID}" /workspace/.pre_entry.sh
+    fi
+
+    # Enable passwordless sudo capabilities for the user
+    chown root:"${CI_BUILD_GID}" "$(which gosu)"
+    chmod +s "$(which gosu)"; sync
+
+    exec gosu "${CI_BUILD_UID}:${CI_BUILD_GID}" "${COMMAND[@]}"
+else
+    exec "${COMMAND[@]}"
+fi