Add a "one shot" mode for just doing the upgrade

As per the request:

  #13

README.md

@@ -1,45 +1,61 @@
-This is a PostgreSQL Docker container that automatically
-upgrades your database.
+This is a PostgreSQL Docker image to automatically upgrade
+your database.
 
-It's whole purpose in life is to automatically detect the
+Its whole purpose in life is to automatically detect the
 version of PostgreSQL used in the existing PostgreSQL data
-directory, and automatically upgrade it (if needed) to the
+directory, then automatically upgrade it (if needed) to the
 required version of PostgreSQL.
 
 After this, the PostgreSQL server starts and runs as per
 normal.
 
-The reason this Docker container is needed, is because
-the official Docker PostgreSQL container has no ability
-to handle version upgrades, which leaves people to figure
-it out manually (not great): https://github.com/docker-library/postgres/issues/37
+The reason this Docker image is needed, is because the
+official Docker PostgreSQL image has no ability to handle
+version upgrades, which leaves people to figure it out
+manually (not great): https://github.com/docker-library/postgres/issues/37
 
 ## WARNING! Backup your data!
 
-This Docker container does an in-place upgrade of the database
+This Docker image does an in-place upgrade of the database
 data, so if something goes wrong you are expected to already
 have backups you can restore from.
 
-## How to use this container
+## How to use this image
 
-This container is on Docker Hub:
+This image is on Docker Hub:
 
 https://hub.docker.com/r/pgautoupgrade/pgautoupgrade
 
-To always use the latest version of PostgreSQL, use
-the tag `latest`:
+To always use the latest version of PostgreSQL, use the tag
+`latest`:
 
     pgautoupgrade/pgautoupgrade:latest
 
 If you instead want to run a specific version of PostgreSQL
-then pick a matching tag on our Docker Hub.  For example,
-to use PostgreSQL 15 you can use:
+then pick a matching tag on our Docker Hub.  For example, to
+use PostgreSQL 15 you can use:
 
     pgautoupgrade/pgautoupgrade:15-alpine3.8
 
+### "One shot" mode
+
+If you just want to perform the upgrade without starting PostgreSQL
+afterwards, then you can use "[One Shot](https://github.com/pgautoupgrade/docker-pgautoupgrade/issues/13)" mode.
+
+To do that, add an environment variable called `PGAUTO_ONESHOT`
+(equal to `yes`) when you run the container.  Like this:
+
+```
+$ docker run --name pgauto -it \
+		--mount type=bind,source=/path/to/your/database/directory,target=/var/lib/postgresql/data \
+		-e POSTGRES_PASSWORD=password \
+		-e PGAUTO_ONESHOT=yes \
+		<NAME_OF_THE_PGAUTOUPGRADE_IMAGE>
+```
+
 # For Developers
 
-## Building the container
+## Building the image
 
 To build the development docker image, use:
 
@@ -48,14 +64,14 @@ $ make dev
 ```
 
 This will take a few minutes to create the "pgautoupgrade:latest"
-docker container, that you can use in your docker-compose.yml
+docker image, that you can use in your docker-compose.yml
 files.
 
-## Breakpoints in the container
+## Breakpoints in the image
 
 There are (at present) two predefined er... "breakpoints"
-in the container.  When you run the container with either
-of them, then the container will start up and keep running,
+in the image.  When you run the image with either
+of them, then the image will start up and keep running,
 but the docker-entrypoint script will pause at the chosen
 location.
 
@@ -83,7 +99,7 @@ PostgreSQL acting on them.
 $ make server
 ```
 
-## Testing the container image
+## Testing the image
 
 To run the tests, use:
 
@@ -93,12 +109,13 @@ $ make test
 
 The test script creates an initial PostgreSQL database for
 Redash using an older PG version, then starts Redash using
-the above "automatic updating" PostgreSQL container to
-update the database to the latest PostgreSQL version.
+the above "automatic updating" PostgreSQL image to update
+the database to the latest PostgreSQL version.
 
 It then checks that the database files were indeed updated
 to the newest PostgreSQL release, and outputs an obvious
 SUCCESS/FAILURE message for that loop.
 
-The test runs in a loop, testing (in sequence) PostgreSQL
-versions 9.5, 9.6, 10.x, 11.x, 12.x, 13.x, and 14.x.
+The test runs in a loop, testing (in sequence) upgrades from
+PostgreSQL versions 9.5, 9.6, 10.x, 11.x, 12.x, 13.x, 14.x
+and 15.x.

docker-entrypoint.sh

@@ -566,7 +566,13 @@ _main() {
 			sleep 5
 		done
 	else
-		exec "$@"
+		if [ "x${PGAUTO_ONESHOT}" = "xyes" ]; then
+			echo "*****************************************************************************************************"
+			echo "'One shot' automatic upgrade was requested, so exiting now rather than starting the PostgreSQL server"
+			echo "*****************************************************************************************************"
+		else
+			exec "$@"
+		fi
 	fi
 }