infra: Make docker folder read only, remove unused tests. (#384)

* Remove container tests.
* Rename buildspec.yml -> buildspec-container-pr.yml.
* Rename buildspec-toolkit.yml -> buildspec.yml.
* Rename test-toolkit/ -> test/.
* Refactor tests.

Author: nadiaya

.flake8

@@ -1,3 +1,3 @@
 [flake8]
-application_import_names = image_utils, integration, sagemaker_tensorflow_container, test, test-toolkit, timeout, utils
+application_import_names = image_utils, integration, sagemaker_tensorflow_container, test, timeout, utils
 import-order-style = google

buildspec-container-pr.yml

@@ -0,0 +1,13 @@
+version: 0.2
+
+phases:
+  pre_build:
+    commands:
+      - PR_NUM=$(echo $CODEBUILD_SOURCE_VERSION | grep -o '[0-9]\+')
+      - echo 'Pull request number:' $PR_NUM '. No value means this build is not from pull request.'
+
+  build:
+    commands:
+
+      - error_cmd="echo 'In order to make changes to the docker files, please, use https://github.com/aws/deep-learning-containers repository.' && exit 1"
+      - execute-command-if-has-matching-changes "$error_cmd" "docker/"