Marina

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- [build] Fix an issue when the system does not already have the busybox
image installed.

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- [build] Fix some unicode issues when running on Python 2.

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- [build] Add another workaround for docker-py when pulling a new image.
See https://github.com/docker/docker-py/issues/1134

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- Support Python 3.

- [build] Stop using an API that was removed in docker 1.12.

- [build] Allow the BUILD_CONTEXT (cwd in scripts) to be writeable.
Previously it was mounted readonly.

- [build] Search for the ssh identity file in known paths.

- [build] Add a workaround for a bug in docker-py causing the runner image
to fail to build. See https://github.com/docker/docker-py/issues/1134

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- [build] Flush stdout to keep container messages from pausing.

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- [build] Delete any volumes associated with a container.
- [build] Fix usage of ExposedPorts from runner base image.