I searched for the healthcheck/postgres:alpine and I found the following repository with the Dockerfile:
Assuming this is the correct repo for your docker image, the dockerfile specifies:
COPY docker-healthcheck /usr/local/bin/
HEALTHCHECK CMD ["docker-healthcheck"]
So it extends the postgres docker image which is documented here:
Specifically for the password they mention the following:
POSTGRES_PASSWORD This environment variable is recommended for you to
use the PostgreSQL image. This environment variable sets the superuser
password for PostgreSQL. The default superuser is defined by the
POSTGRES_USER environment variable.
Note 1: The PostgreSQL image sets up trust authentication locally so
you may notice a password is not required when connecting from
localhost (inside the same container). However, a password will be
required if connecting from a different host/container...
Read the whole documentation at the image homepage.