Add an ADR for using a single docker image for all deployments (#9123)

Re https://github.com/mozmeao/infra/issues/1306

[ci skip]

docs/architecture/decisions/0005-use-a-single-docker-image-for-all-deployments.md

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+# 5. Use a Single Docker Image For All Deployments
+
+Date: 2020-07-07
+
+## Status
+
+Accepted
+
+## Context
+
+We currently build an individual docker image for each deployment (dev, stage, and prod) that contains the
+proper data for that environment. It would save time and testing if we only built a single image that could
+be promoted to each environment and loaded with the proper data at startup.
+
+## Decision
+
+We will use a Kubernetes DaemonSet to ensure that a data updater pod is running on each node in a cluster. This
+pod will keep the database and l10n files updated in a volume that will be used by the other bedrock pods to
+access the data.
+
+[GitHub issue](https://github.com/mozmeao/infra/issues/1306)
+
+## Consequences
+
+This change means that bedrock will be more simple to run because each pod will no longer need to be responsible for
+keeping its data updated, and so it will run only the bedrock web process and not also the updater daemon. It also
+means that there is a risk of a bedrock pod being run on a node that hasn't had the updater pod run yet, so there
+would be no available data. We will handle this by ensuring that bedrock won't start when the data isn't available,
+and so k8s will not send traffic to those pods until they're successfully up and responding, and will keep trying
+to start pods on the node untill they succeed.