nginx-alpine: how to view its docker file

Question

I have the following image: nginx:1.19.0-alpine

I want to know about its docker file. I checked https://github.com/nginxinc/docker-nginx but could not understand how to check

Basically i want to change the /docker-entrypoint.sh

Currently its

#!/usr/bin/env sh
# vim:sw=4:ts=4:et

set -e

if [ "$1" = "nginx" -o "$1" = "nginx-debug" ]; then
    if /usr/bin/find "/docker-entrypoint.d/" -mindepth 1 -maxdepth 1 -type f -print -quit 2>/dev/null | read v; then
        echo "$0: /docker-entrypoint.d/ is not empty, will attempt to perform configuration"

        echo "$0: Looking for shell scripts in /docker-entrypoint.d/"
        find "/docker-entrypoint.d/" -follow -type f -print | sort -n | while read -r f; do
            case "$f" in
                *.sh)
                    if [ -x "$f" ]; then
                        echo "$0: Launching $f";
                        "$f"
                    else
                        # warn on shell scripts without exec bit
                        echo "$0: Ignoring $f, not executable";
                    fi
                    ;;
                *) echo "$0: Ignoring $f";;
            esac
        done

        echo "$0: Configuration complete; ready for start up"
    else
        echo "$0: No files found in /docker-entrypoint.d/, skipping configuration"
    fi
fi

# Handle enabling SSL
if [ "$ENABLE_SSL" = "True" ]; then
  echo "Enabling SSL support!"
  cp /etc/nginx/configs/default_ssl.conf /etc/nginx/conf.d/default.conf
fi


exec "$@"

I want it to modify it as

#!/usr/bin/env sh
# vim:sw=4:ts=4:et

set -x -o verbose;

echo $1

if [ "$1" = "nginx" -o "$1" = "nginx-debug" ]; then
    if /usr/bin/find "/docker-entrypoint.d/" -mindepth 1 -maxdepth 1 -type f -print -quit 2>/dev/null | read v; then
        echo "$0: /docker-entrypoint.d/ is not empty, will attempt to perform configuration"

        echo "$0: Looking for shell scripts in /docker-entrypoint.d/"
        find "/docker-entrypoint.d/" -follow -type f -print | sort -n | while read -r f; do
            case "$f" in
                *.sh)
                    if [ -x "$f" ]; then
                        echo "$0: Launching $f";
                        "$f"
                    else
                        # warn on shell scripts without exec bit
                        echo "$0: Ignoring $f, not executable";
                    fi
                    ;;
                *) echo "$0: Ignoring $f";;
            esac
        done

        echo "$0: Configuration complete; ready for start up"
    else
        echo "$0: No files found in /docker-entrypoint.d/, skipping configuration"
    fi
fi

# Handle enabling SSL
if [ "$ENABLE_SSL" = "True" ]; then
  echo "Enabling SSL support!"
  cp /etc/nginx/configs/default_ssl.conf /etc/nginx/conf.d/default.conf
fi

exec "$@"

I see $1 is being passed. But no clue what to pass