Add API tokens for new Hetzner cloud sandbox project

A new Hetzner cloud project has been created called "Sandbox". This
project is meant for non-production workload which must be created
on-demand from e.g. a CI pipeline. The first project using the sandbox
is aurweb, which wants to use GitLab's Review apps[1] feature to create
dynamic environments on-demand.

Two API tokens have been created, one for the infrastructure project (to
be used by packer) and for the aurweb project.

[1] https://docs.gitlab.com/ee/ci/review_apps/

README.md

@@ -49,6 +49,10 @@ In order to use this, you need to install packer and then run
 
 This will take some time after which a new snapshot will have been created on the primary hcloud archlinux project.
 
+For the sandbox project please run
+
+    packer build -var $(misc/get_key.py misc/vaults/vault_hetzner.yml hetzner_cloud_sandbox_infrastructure_api_key --format env | sed 's/_sandbox_infrastructure//') packer/archlinux.pkr.hcl
+
 #### Note about terraform
 
 We use terraform in two ways:

misc/vaults/vault_hetzner.yml

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