Importing self-signed cert into Docker’s JRE cacert is not recognized by the service

Hence imported the self-signed certificate of HTTPS external URL into Docker container’s JRE cacert keystore.

No: you need to import it into the Docker image from which you run your container.

Importing it into the container would only create a temporary writable data layer, which will be discarded when you restart your container.

Something like this answer:

USER root
COPY ldap.cer $JAVA_HOME/jre/lib/security
RUN \
    cd $JAVA_HOME/jre/lib/security \
    && keytool -keystore cacerts -storepass changeit -noprompt -trustcacerts -importcert -alias ldapcert -file ldap.cer