diff --git a/rootfs/root/docker/setup/05-custom.sh b/rootfs/root/docker/setup/05-custom.sh
index 2a22d6e..0042a61 100755
--- a/rootfs/root/docker/setup/05-custom.sh
+++ b/rootfs/root/docker/setup/05-custom.sh
@@ -22,6 +22,9 @@
 # Set bash options
 set -eo pipefail
 [ "$DEBUGGER" = "on" ] && echo "Enabling debugging" && set -x$DEBUGGER_OPTIONS
+# Force IPv4 for all curl calls in this script — the base image IPv6 routing
+# intercepts *.github.com and presents a cert for casjay.in, causing SAN mismatch
+printf -- '-4\n' > /root/.curlrc
 # - - - - - - - - - - - - - - - - - - - - - - - - -
 # Set env variables
 exitCode=0