fix: surface podman errors instead of aborting silently

`set -e` aborted build.sh when `podman image rm`/`prune` failed, and
`2>/dev/null` hid the reason, so --remove exited 125 with no output.
Combining --remove --prune also skipped the prune.

Guard the removal with `podman image exists` for a clear "nothing to
remove", drop `2>/dev/null` so podman's reason is visible, and use
`cmd || echo` to warn without aborting. Exit status stays 0 on failure
by design.

build.sh

@@ -42,10 +42,14 @@ fi
 
 if ((REMOVE || PRUNE)); then
     if ((REMOVE)); then
-        podman image rm "${IMAGE}" 2>/dev/null
+        if podman image exists "$IMAGE"; then
+            podman image rm "${IMAGE}" || echo "WARNING: could not remove $IMAGE" >&2
+        else
+            echo "$IMAGE does not exist, nothing to remove"
+        fi
     fi
     if ((PRUNE)); then
-        podman image prune -f 2>/dev/null
+        podman image prune -f || echo "WARNING: prune failed" >&2
     fi
     exit 0
 fi