Fix build: use renamed PocketBase checksums.txt asset

PocketBase renamed its release checksums asset from
pocketbase_<version>_checksums.txt to checksums.txt. The old URL now
returns 404, which fails BuildKit's ADD cache-key request with
"failed to load cache key: invalid response status 404".

Point the ADD in both Dockerfiles at the new checksums.txt name. The
file's contents still list entries by full zip filename, so the
sha256sum verification is unaffected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Dockerfile.rootless

@@ -11,7 +11,7 @@ RUN apk add --no-cache unzip ca-certificates
 
 # download the release zip and its checksums file
 ADD https://github.com/pocketbase/pocketbase/releases/download/v${PB_VERSION}/pocketbase_${PB_VERSION}_linux_${TARGETARCH}.zip /tmp/pb.zip
-ADD https://github.com/pocketbase/pocketbase/releases/download/v${PB_VERSION}/pocketbase_${PB_VERSION}_checksums.txt /tmp/pb_checksums.txt
+ADD https://github.com/pocketbase/pocketbase/releases/download/v${PB_VERSION}/checksums.txt /tmp/pb_checksums.txt
 
 # verify integrity against the published checksums, then unzip
 RUN grep "pocketbase_${PB_VERSION}_linux_${TARGETARCH}.zip" /tmp/pb_checksums.txt \