build: use trampoline_v2 for python samples and allow custom dockerfile (#881)

* build: use trampoline_v2 for python samples and allow custom dockerfile

Source-Link: https://github.com/googleapis/synthtool/commit/a7ed11ec0863c422ba2e73aafa75eab22c32b33d
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:6e7328583be8edd3ba8f35311c76a1ecbc823010279ccb6ab46b7a76e25eafcc

* chore: add trampolinerc

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Bu Sun Kim <[email protected]>
diff --git a/owlbot.py b/owlbot.py
index 58aa53a..611ce92 100644
--- a/owlbot.py
+++ b/owlbot.py
@@ -17,6 +17,10 @@
         "build.sh",
     ],
 )  # just move kokoro configs
+s.move(
+    # needed by samples kokoro jobs
+    templated_files / ".trampolinerc"
+)
 
 
 assert 1 == s.replace(