[Bugfix] Fix openEuler dockerfile error (#6871)

### What this PR does / why we need it?
This pull request addresses several issues within the openEuler
Dockerfiles : `Dockerfile.a3.openEuler` and `Dockerfile.openEuler` to
ensure correct installation and setup of the Mooncake dependency. The
changes primarily involve fixing incorrect file paths for the
mooncake_installer.sh script and streamlining the declaration of the
MOONCAKE_TAG build argument, leading to more robust and accurate
container builds.
### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: v0.16.0
- vLLM main:
15d76f74e2

Signed-off-by: wjunLu <wjunlu217@gmail.com>

.github/workflows/_schedule_image_build.yaml

@@ -89,8 +89,10 @@ jobs:
           fi
           branch="${branch:-main}"
         else
-          # For branch push / schedule / workflow_dispatch, use the triggering branch name
-          branch="${{ github.ref_name }}"
+          # For PR events github.ref_name is "<PR_number>/merge" which has no cached image;
+          # use base_ref (target branch) instead. For push/schedule, base_ref is empty so
+          # fall back to ref_name which is the actual branch name.
+          branch="${{ github.base_ref || github.ref_name }}"
         fi
         # Replace / with - for use in image tags
         branch="${branch//\//-}"