build: use trampoline_v2 for python samples and allow custom dockerfile (#1552)

diff --git a/owlbot.py b/owlbot.py
index 1df7f43..1ebdf88 100644
--- a/owlbot.py
+++ b/owlbot.py
@@ -27,6 +27,7 @@
 # Copy kokoro configs.
 # Docs are excluded as repo docs cannot currently be generated using sphinx.
 s.move(templated_files / '.kokoro', excludes=['**/docs/*', 'publish-docs.sh'])
+s.move(templated_files / '.trampolinerc')  # config file for trampoline_v2
 
 # Also move issue templates
 s.move(templated_files / '.github')