Install + set up PgBackRest in postgresql-base

[pypt]

Install and configure PgBackRest to postgresql-base so that images that derive from it - postgresql-server and temporal-postgresql - could use it.

The only supported repository for backups in this PR is Amazon S3 and S3-compatible object stores. Repository's credentials are configured through (optional) environment variables.

There's a terse doc on how to do full / incremental backups too.

As per #812, I intend to deploy this, do a full backup, see how long does it take + how much space does it occupy, and then come up with a backup strategy.

Unrelated, but in:

https://github.com/mediacloud/backend/compare/pgbackrest?expand=1#diff-2c08109dfb859a1eab9191db0d9aab0b4b330e09c5666f15e5654ca0be3b6933R1930-R1936

/dev/shm's size for temporal-postgresql gets increased too. PostgreSQL used to rely on /dev/shm heavily, I'm not sure if that's the case anymore, but it doesn't hurt to hike it up a bit.

References #812.

[jtotoole]

oh also, from where should these commands be run? woodward?

[pypt]

Currently woodward, but the official definition is "whichever server has a role-postgresql-server Docker Swarm label set to it in production Ansible inventory (hosts.yml)".

Also it has to be run within a running postgresql-server container.

[jtotoole]

This looks really good! Questions just for my understanding:

• From where does one configure backup frequency?
• Is there a way to conditionally increase MC_PGBACKREST_RETENTION_FULL if the latest backup has failed? Is that something we'd want to do?

[pypt]

From where does one configure backup frequency?

At the moment backups happen when we run pgbackrest backup manually. Later when we know long do they take and how much disk space does a typical backup occupy, we'll add them as a Cron job somewhere.

Is there a way to conditionally increase MC_PGBACKREST_RETENTION_FULL if the latest backup has failed? Is that something we'd want to do?

Could you clarify this question?

[pypt]

Merging this one then, but let me know if you have any more improvements to suggest / questions.