Change PGDATA in 17+ to /var/lib/postgresql/MAJOR/docker

This is a pretty large breaking change, which is why this only makes the change in 17+ (which is currently in pre-release stages, and not due for GA until September, and pre-release `PGDATA` directories are officially *not supported* on the GA release anyhow).

Concretely, this changes `PGDATA` to `/var/lib/postgresql/MAJOR/docker`, which matches the pre-existing convention/standard of the `pg_ctlcluster`/`postgresql-common` set of commands, and frankly is what we should've done to begin with, in a classic case of Chesterton's Fence (https://en.wikipedia.org/wiki/Wikipedia:Chesterton%27s_fence).

This also changes the `VOLUME` to `/var/lib/postgresql`, which should be more reasonable, and make the upgrade constraints more obvious.

For any users who have been testing the pre-releases, the simplest way to keep your existing data directory is going to be to adjust your bind-mount from `/var/lib/postgresql/data` to `/var/lib/postgresql/17/docker`, but the *best* way is going to be to refactor your host directory such that your data lives at `17/docker` inside and you can then mount directly to `/var/lib/postgresql`.

Dockerfile-alpine.template

@@ -209,10 +209,19 @@ RUN set -eux; \
 
 RUN install --verbose --directory --owner postgres --group postgres --mode 3777 /var/run/postgresql
 
+{{ if .major >= 17 then ( -}}
+#
+# NOTE: in 17+, PGDATA has changed to match the pg_ctlcluster standard directory structure, and the VOLUME has moved from /var/lib/postgresql/data to /var/lib/postgresql
+#
+ENV PGDATA /var/lib/postgresql/{{ .major | tostring }}/docker
+VOLUME /var/lib/postgresql
+# ("/var/lib/postgresql" is already pre-created with suitably usable permissions above)
+{{ ) else ( -}}
 ENV PGDATA /var/lib/postgresql/data
 # this 1777 will be replaced by 0700 at runtime (allows semi-arbitrary "--user" values)
 RUN install --verbose --directory --owner postgres --group postgres --mode 1777 "$PGDATA"
 VOLUME /var/lib/postgresql/data
+{{ ) end -}}
 
 COPY docker-entrypoint.sh docker-ensure-initdb.sh /usr/local/bin/
 RUN ln -sT docker-ensure-initdb.sh /usr/local/bin/docker-enforce-initdb.sh

Dockerfile-debian.template

@@ -180,10 +180,19 @@ RUN set -eux; \
 
 RUN install --verbose --directory --owner postgres --group postgres --mode 3777 /var/run/postgresql
 
+{{ if .major >= 17 then ( -}}
+#
+# NOTE: in 17+, PGDATA has changed to match the pg_ctlcluster standard directory structure, and the VOLUME has moved from /var/lib/postgresql/data to /var/lib/postgresql
+#
+ENV PGDATA /var/lib/postgresql/{{ .major | tostring }}/docker
+VOLUME /var/lib/postgresql
+# ("/var/lib/postgresql" is already pre-created with suitably usable permissions above)
+{{ ) else ( -}}
 ENV PGDATA /var/lib/postgresql/data
 # this 1777 will be replaced by 0700 at runtime (allows semi-arbitrary "--user" values)
 RUN install --verbose --directory --owner postgres --group postgres --mode 1777 "$PGDATA"
 VOLUME /var/lib/postgresql/data
+{{ ) end -}}
 
 COPY docker-entrypoint.sh docker-ensure-initdb.sh /usr/local/bin/
 RUN ln -sT docker-ensure-initdb.sh /usr/local/bin/docker-enforce-initdb.sh