Use memory for the /tmp directory

[craddm]

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⤴️ Summary

Mounts tmp using tmpfs, so that it uses memory instead of disk space for temporary files.

Currently uses 50% of RAM, and will use disk space if needed (e.g. files too big). Not sure if it will prevent disk lock-up or how to test that.

🌂 Related issues

Relates to #1071.

🔬 Tests

Deployed an SRD with tmp mounted on tmpfs