Run PR validation check only when needed & use docker cache during build (#876)

* Improve PR validation checks

devops/check_modified_files.sh

@@ -0,0 +1,45 @@
+#!/bin/bash
+paths_arr=( "$@" )
+
+printf "\n========== List modified files ==========\n"
+echo "$(git diff --name-only HEAD^ HEAD)"
+
+printf "\n========== List paths to match and check existence ==========\n"
+for path in ${paths_arr[*]}
+do
+  if [ -f "$path" ] || [ -d "$path" ]; then
+      echo "$path - found"
+  else
+      echo "$path - does not found - exiting with failure"
+      exit 1
+  fi
+done
+
+printf "\n========== Check paths of modified files ==========\n"
+git diff --name-only HEAD^ HEAD > files.txt
+matched=false
+while IFS= read -r file
+do
+  for path in ${paths_arr[*]}
+  do
+      if [[ $file == $path* ]]; then
+        echo "$file - match path: $path"
+        matched=true
+        break
+      fi
+  done
+  if [[ $matched == true ]]; then
+      break
+  else
+      echo "$file - does not match any given path"
+  fi
+done < files.txt
+
+printf "\n========== Result ==========\n"
+if [[ $matched = true ]]; then
+  echo "match found"
+  echo "::set-output name=matched::true"
+else
+  echo "no match found"
+  echo "::set-output name=matched::false"
+fi