It seems like your standalone.xml file was not read and jboss tries to use a default configuration instead.
There is a mention of updating the default filename to standalone-ha.xml here
https://lists.jboss.org/pipermail/keycloak-dev/2018-October/011304.html.
updating the image building command to
COPY ./_resources/standalone.xml /opt/jboss/keycloak/standalone/configuration/standalone-ha.xml
should help
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