Introduced DockerContainerizerProcess::Container::name.

Review: https://reviews.apache.org/r/26611
Introduced DockerContainerizerProcess::Container::create.

Simplifies the generic setup of a new container that currently
consists of making sure stdout/stderr exists and the directory is
properly chown'ed.

Review: https://reviews.apache.org/r/26612
Simplified launch continuations in DockerContainerizerProcess.

Combined redundant code when launching a Docker container with and
without a task.

Review: https://reviews.apache.org/r/26613
More simplifications of Docker container launching.

Review: https://reviews.apache.org/r/26614
Even more simplification of launch paths in Docker containerizer.

Review: https://reviews.apache.org/r/26615
Minor cleanup when creating Docker containers.

Review: https://reviews.apache.org/r/26616
And more refactoring in the Docker containerizer to simplify.

Review: https://reviews.apache.org/r/26617
Removed unnecessary parameter.

Review: https://reviews.apache.org/r/26618
Use the right resources when starting Docker container.

Review: https://reviews.apache.org/r/26619
Removed redundant environment for Docker containers.

Review: https://reviews.apache.org/r/26620
Replaced all tabs with spaces.

Review: https://reviews.apache.org/r/26621
Fixed non-deterministic test.

The DockerContainerizerTest.ROOT_DOCKER_PortMapping test was doing a
Docker::kill on the container which should be finishing properly which
was causing the call to Docker::kill to fail when the container had
already finished properly and been cleaned up, thus causing the test
to fail.

Review: https://reviews.apache.org/r/26630