seems to work a lot better

base/entrypoint.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+
+# Start the ssh-agent if it's not already running
+if [ ! -S /tmp/ssh-agent.sock ]; then
+    eval $(ssh-agent -a /tmp/ssh-agent.sock)
+fi
+
+# Export environment variables globally
+export SSH_AUTH_SOCK=/tmp/ssh-agent.sock
+echo "export SSH_AUTH_SOCK=/tmp/ssh-agent.sock" >> /etc/profile.d/ssh-agent.sh
+
+# Optionally preload keys
+# ssh-add /path/to/id_rsa
+
+# Start tmux
+SSH_AUTH_SOCK=/tmp/ssh-agent.sock tmux