open-coredump: pass the content of "image" file not its path to dbuild

in a4eb3c6e0f, we passed the path of
"image" to `dbuild`, but that was wrong. we should pass its content
to this script. so in this change, it is fixed.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes #15247

scripts/open-coredump.sh

@@ -344,8 +344,9 @@ EOF
 fi
 
 TOP_SRCDIR="$(dirname "$(dirname "$(readlink -f "$0")")")"
+IMAGE="${SCYLLA_REPO_PATH}/tools/toolchain/image"
 exec ${TOP_SRCDIR}/tools/toolchain/dbuild               \
-    --image ${SCYLLA_REPO_PATH}/tools/toolchain/image   \
+    --image "$(<"$IMAGE")"                              \
     -it                                                 \
     -v $(pwd):/workdir                                  \
     -v ${SCYLLA_REPO_PATH}:/src/scylla                  \