In the dockerfile to add Web Terminal I had to combine the two RUN commands into one line to make it behave properly.  The ccontrol start seemed to complete within the RUN command.

# Start Caché Instance
# Generate login and password for csession if needed, and Load downloaded xml with compilation 
# WebTerminal will be installed during compilation process

RUN ccontrol start $ISC_PACKAGE_INSTANCENAME && printf "_SYSTEM\nSYS\n" \
 |  csession $ISC_PACKAGE_INSTANCENAME -UUSER "##class(%SYSTEM.OBJ).Load(\"/tmp/webterminal/webterminal.xml\",\"cdk\")"