Bug 1504906 - Use a separate docker image for base toolchain builds. r=gps

We're going to bump our shipped builds to build against Gtk+ 3.10, but
still want to ensure we can still build against Gtk+ 3.4. As we're using
Gtk+ packages installed in the build docker image, we need to have a
separate image where the Gtk+ packages are kept at version 3.4.

Differential Revision: https://phabricator.services.mozilla.com/D11137

taskcluster/ci/build/linux.yml

@@ -249,6 +249,7 @@ linux64-base-toolchains/opt:
         symbol: Bb
     worker-type: aws-provisioner-v1/gecko-{level}-b-linux
     worker:
+        docker-image: {in-tree: debian7-amd64-build-base}
         max-run-time: 7200
         env:
             PERFHERDER_EXTRA_OPTIONS: base-toolchains

taskcluster/ci/docker-image/kind.yml

@@ -44,6 +44,14 @@ jobs:
     packages:
       - deb7-cmake
       - deb7-ninja
+  debian7-amd64-build-base:
+    symbol: I(deb7-bb)
+    parent: debian7-base
+    definition: debian7-build
+    packages:
+      - deb7-valgrind
+    args:
+      ARCH: amd64
   debian7-amd64-build:
     symbol: I(deb7)
     parent: debian7-base