build: use trampoline_v2 for python samples and allow custom dockerfile (#267)

Source-Link: googleapis/synthtool@a7ed11e
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:6e7328583be8edd3ba8f35311c76a1ecbc823010279ccb6ab46b7a76e25eafcc

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: gcf-owl-bot[bot]

packages/google-cloud-python-speech/.trampolinerc

@@ -16,15 +16,26 @@
 
 # Add required env vars here.
 required_envvars+=(
-    "STAGING_BUCKET"
-    "V2_STAGING_BUCKET"
 )
 
 # Add env vars which are passed down into the container here.
 pass_down_envvars+=(
+    "NOX_SESSION"
+    ###############
+    # Docs builds
+    ###############
     "STAGING_BUCKET"
     "V2_STAGING_BUCKET"
-    "NOX_SESSION"
+    ##################
+    # Samples builds
+    ##################
+    "INSTALL_LIBRARY_FROM_SOURCE"
+    "RUN_TESTS_SESSION"
+    "BUILD_SPECIFIC_GCLOUD_PROJECT"
+    # Target directories.
+    "RUN_TESTS_DIRS"
+    # The nox session to run.
+    "RUN_TESTS_SESSION"
 )
 
 # Prevent unintentional override on the default image.