fix(build): lowercase app-id for podman image tag names

podman rejects repository names with uppercase letters. App-ids like
io.github.DenysMb.Kontainer contain uppercase, causing 'repository name
must be lowercase' (exit 125) in the chunkah split step.

Derive APP_LOWER=${APP,,} and use it for all localhost image tags while
keeping APP (original case) for filesystem paths and OCI artifact names.

Assisted-by: Claude Sonnet 4.6 via OpenCode

Author: castrojo

.github/workflows/build.yml

@@ -480,9 +480,11 @@ jobs:
           APP: ${{ matrix.app }}
           CHUNKAH_SPLITTER: https://github.com/coreos/chunkah/releases/download/v0.3.0/Containerfile.splitter
         run: |
-          INPUT="localhost/${APP}:input"
-          LABELED="localhost/${APP}:labeled"
-          CHUNKED="localhost/${APP}:chunked"
+          # podman requires repository names to be lowercase; app-ids may contain uppercase
+          APP_LOWER="${APP,,}"
+          INPUT="localhost/${APP_LOWER}:input"
+          LABELED="localhost/${APP_LOWER}:labeled"
+          CHUNKED="localhost/${APP_LOWER}:chunked"
 
           # Pull OCI dir into the shared podman/buildah store and tag for stable reference.
           IMAGE_ID=$(podman pull --quiet "oci:./${APP}.oci")