Finish building Postgres support (#133)

This includes:
 - Building a Docker image for Postgres as well as SQLite
 - Fuller instructions for usage of the package, including the Postgres builds.

A few related things changed here:
 - `.env` is not used anymore -- the defaults in the Dockerfiles are
   sufficient
 - The Rust version in the Dockerfiles is increased to match the MSRV,
   and with it the Alpine version bumped to one built with that Rust
   version.
 - Cargo dependencies on native-tls and openssl updated to include only
   the `vendored` feature, so as not to require a system openssl
   installation.
 - Two GitHub jobs are set up, to build the two different Docker images
 - The documentation incorrectly suggested using `DELETE .. CASCADE` to
   delete clients. This syntax does not exist, as the cascading delete
   is configured in the schema.

docs/src/integration/pre-built.md

@@ -0,0 +1,40 @@
+# Pre-built Images
+
+The pre-built Postgres Docker image described in [Docker
+Images](../usage/docker-images.md) may be adequate for a production deployment.
+The image is stateless and can be easily scaled horizontally to increase
+capacity.
+
+## Database Schema
+
+The schema defined in
+[`postgres/schema.sql`](https://github.com/GothenburgBitFactory/taskchampion-sync-server/blob/main/postgres/schema.sql)
+must be applied to the database before the container will function.
+
+The schema is stable, and any changes to the schema will be made in a major
+version with migration instructions provided.
+
+An integration may:
+
+- Add additional tables to the database
+- Add additional columns to the `clients` table. If those columns do not have
+default values, ensure the server is configured with `CREATE_CLIENTS=false` as
+described below.
+- Insert rows into the `clients` table, using default values for all columns
+except `client_id` and any application-specific columns.
+- Delete rows from the `clients` table. Note that this table is configured to
+automatically delete all data associated with a client when the client's row is
+deleted.
+
+## Managing Clients
+
+By default, taskchampion-sync-server creates a new, empty client when it
+receives a connection from an unrecognized client ID. Setting
+`CREATE_CLIENTS=false` disables this functionality, and is recommended in
+production deployments to avoid abuse.
+
+In this configuration, it is the responsibility of the integration to create
+new client rows when desired, using a statement like `INSERT into clients
+(client_id) values ($1)` with the new client ID as a parameter. Similarly,
+clients may be deleted, along with all stored task data, using a statement like
+`DELETE from clients where client_id = $1`.