Improvement: don't attempt to execute XML files

[lower29]

This addresses issue #11. The actual fix is in https://github.com/artefactual/automation-tools/pull/43 so this remains work-in-progress until that PR has been merged into the base artefactual/automation-tools. Once that happens, the automation-tools submodule can be updated to point to the right commit so that the required functionality is built into our Docker image.

There are some additional changes to this repo:

• Added base packages to apk install, because these are used to build some of the packages specified in the requirements.txt for the Automation Tools
• Made the log level configurable via the AM_TOOLS_TRANSFER_LOG_LEVEL environment variable

The first of these is required for a successful build, the latter just makes it easier to debug things.

As mentioned, this PR shouldn't be approved or merged until the proposed change to artefactual/automation-tools has been approved and merged.

[lower29]

Thanks @jhsimpson I've updated the automation-tools submodule to the v0.1.0-rc2 and merged.