rockylinux:8 libswoc 1.5.15 (#391)

diff --git a/docker/rockylinux8/Dockerfile b/docker/rockylinux8/Dockerfile
index 0ea3221..fb9d4ae 100644
--- a/docker/rockylinux8/Dockerfile
+++ b/docker/rockylinux8/Dockerfile
@@ -257,7 +257,7 @@
   cd /root/
   mkdir libswoc
   cd libswoc
-  swoc_version=1.5.13
+  swoc_version=1.5.15
 
   # Now, checkout that version and install libswoc in /opt/libswoc
   git clone https://github.com/apache/trafficserver-libswoc.git