Due to the Keycloak image being modified to enhance security and curl being removed. The below bash script provides an alternative method for providing a healthcheck. The Dockerfile and docker-compose.yml files contain the snippets to enable the healthchck

docker-compose.yml

healthcheck:
  test: [
    "CMD-SHELL", 
    "bash /opt/keycloak/bin/docker-healthcheck.sh"
  ]
  interval: 30s
  timeout: 5s
  retries: 3

docker-healthcheck.sh

#!/bin/bash

# Opens a network socket connection to localhost on port 8080 and associates
# it with file descriptor 3. This allows reading from and writing to the
# socket using that file descriptor.
exec 3<>/dev/tcp/localhost/8080

# Sends an HTTP GET request to the server running on localhost at port 8080.
# The request is sent through the socket using file descriptor 3.
echo -e "GET /health/ready HTTP/1.1\nhost: localhost:8080\n" >&3

# Reads the response from the socket using file descriptor 3 and prints it to
# stdout. The response is then piped to grep which searches for the string
# "status" and "UP" in the response. The grep command exits with status 0 if
# the string is found and 1 if it is not found.
timeout --preserve-status 1 cat <&3 | grep -m 1 status | grep -m 1 UP

# Saves the exit status of the previous command (grep) in the variable ERROR.
ERROR=$?

# Closes the socket. This is important because the script will hang if the
# socket is not closed.
exec 3<&-
exec 3>&-

# Exits with the same status as the previous command.
exit $ERROR

Dockerfile

# Enable health endpoint.
ENV KC_HEALTH_ENABLED=true

# Copies the docker-healthcheck.sh file into the /opt/keycloak directory.
COPY --chown=keycloak:keycloak \
  ./docker-healthcheck.sh \
  /opt/keycloak/bin/docker-healthcheck.sh

# Add healthcheck to execute a shell command as the health check
HEALTHCHECK --interval=30s --timeout=10s --retries=3 \
  CMD-SHELL bash /opt/keycloak/bin/docker-healthcheck.sh