Kokoro: add a new dependency to container image & small tweaks.

More specifically:
* within the container, copy the source tree as root to not depend on the
source tree having o+r permissions (simplifies local invocations).
* define MSAN symbolizer env variable in "standalone tests" script.
* check in the dockerfile.
* bake "pip" into the Docker image as it is now necessary for trace_processor
synthesised tests (see aosp/795694).

The container invocation script is becoming ugly, suggestions welcomed (though
I do not want to bake in an entrypoint script, as the container is also useful
on its own as a "here's all the tools" image).

Change-Id: I63173c6162f05417062d829ad0fa593e64e8e6c4