[SOLVED] Mattermost Docker: WARN[0000] (...) variable is not set

I'm currently trying to set up Mattermost on Docker following the official guide.

After copying and adjusting the .env-file, and while deploying the container (using docker-compose.without-nginx as I'm going to set up traefik as the reverse proxy):

sudo docker-compose -f docker-compose.yml -f docker-compose.without-nginx.yml up -d

the following error message is returned:

WARN[0000] The "POSTGRES_IMAGE_TAG" variable is not set. Defaulting to a blank string. WARN[0000] The "RESTART_POLICY" variable is not set. Defaulting to a blank string. WARN[0000] The "POSTGRES_DATA_PATH" variable is not set. Defaulting to a blank string. WARN[0000] The "RESTART_POLICY" variable is not set. Defaulting to a blank string. WARN[0000] The "MATTERMOST_CONFIG_PATH" variable is not set. Defaulting to a blank string. WARN[0000] The "MATTERMOST_DATA_PATH" variable is not set. Defaulting to a blank string. WARN[0000] The "MATTERMOST_LOGS_PATH" variable is not set. Defaulting to a blank string. WARN[0000] The "MATTERMOST_PLUGINS_PATH" variable is not set. Defaulting to a blank string. WARN[0000] The "MATTERMOST_CLIENT_PLUGINS_PATH" variable is not set. Defaulting to a blank string. WARN[0000] The "MATTERMOST_BLEVE_INDEXES_PATH" variable is not set. Defaulting to a blank string. WARN[0000] The "MATTERMOST_CONTAINER_READONLY" variable is not set. Defaulting to a blank string. error while interpolating services.mattermost.read_only: failed to cast to expected type: invalid boolean:

My .env file looks something like this:

# Domain of service
DOMAIN=my-sub.domain.com

# Container settings
## Timezone inside the containers. The value needs to be in the form 'Europe/Berlin'.
## A list of these tz database names can be looked up at Wikipedia
## https://en.wikipedia.org/wiki/List_of_tz_database_time_zones
TZ=Europe/Berlin
RESTART_POLICY=unless-stopped

# Postgres settings
## Documentation for this image and available settings can be found on hub.docker.com
## https://hub.docker.com/_/postgres
## Please keep in mind this will create a superuser and it's recommended to use a less privileged
## user to connect to the database.
## A guide on how to change the database user to a nonsuperuser can be found in docs/creation-of-nonsuperuser.md
POSTGRES_IMAGE_TAG=13-alpine
POSTGRES_DATA_PATH=./volumes/db/var/lib/postgresql/data

POSTGRES_USER=adjustedusername
POSTGRES_PASSWORD=adjustedpassword
POSTGRES_DB=mattermost

# Nginx
## The nginx container will use a configuration found at the NGINX_MATTERMOST_CONFIG. The config aims
## to be secure and uses a catch-all server vhost which will work out-of-the-box. For additional settings
## or changes ones can edit it or provide another config. Important note: inside the container, nginx sources
## every config file inside */etc/nginx/conf.d* ending with a *.conf* file extension.

## Inside the container the uid and gid is 101. The folder owner can be set with
## `sudo chown -R 101:101 ./nginx` if needed.
NGINX_IMAGE_TAG=alpine

## The folder containing server blocks and any additional config to nginx.conf
NGINX_CONFIG_PATH=./nginx/conf.d
NGINX_DHPARAMS_FILE=./nginx/dhparams4096.pem

CERT_PATH=./volumes/web/cert/cert.pem
KEY_PATH=./volumes/web/cert/key-no-password.pem
#GITLAB_PKI_CHAIN_PATH=<path_to_your_gitlab_pki>/pki_chain.pem
#CERT_PATH=./certs/etc/letsencrypt/live/${DOMAIN}/fullchain.pem
#KEY_PATH=./certs/etc/letsencrypt/live/${DOMAIN}/privkey.pem

## Exposed ports to the host. Inside the container 80 and 443 will be used
HTTPS_PORT=443
HTTP_PORT=80

# Mattermost settings
## Inside the container the uid and gid is 2000. The folder owner can be set with
## `sudo chown -R 2000:2000 ./volumes/app/mattermost`.
MATTERMOST_CONFIG_PATH=./volumes/app/mattermost/config
MATTERMOST_DATA_PATH=./volumes/app/mattermost/data
MATTERMOST_LOGS_PATH=./volumes/app/mattermost/logs
MATTERMOST_PLUGINS_PATH=./volumes/app/mattermost/plugins
MATTERMOST_CLIENT_PLUGINS_PATH=./volumes/app/mattermost/client/plugins
MATTERMOST_BLEVE_INDEXES_PATH=./volumes/app/mattermost/bleve-indexes

## Bleve index (inside the container)
MM_BLEVESETTINGS_INDEXDIR=/mattermost/bleve-indexes

## This will be 'mattermost-enterprise-edition' or 'mattermost-team-edition' based on the version of Mattermost you're installing.
MATTERMOST_IMAGE=mattermost-team-edition
MATTERMOST_IMAGE_TAG=7.1

## Make Mattermost container readonly. This interferes with the regeneration of root.html inside the container. Only use
## it if you know what you're doing.
## See https://github.com/mattermost/docker/issues/18
MATTERMOST_CONTAINER_READONLY=false

## The app port is only relevant for using Mattermost without the nginx container as reverse proxy. This is not meant
## to be used with the internal HTTP server exposed but rather in case one wants to host several services on one host
## or for using it behind another existing reverse proxy.
APP_PORT=8065

## Configuration settings for Mattermost. Documentation on the variables and the settings itself can be found at
## https://docs.mattermost.com/administration/config-settings.html
## Keep in mind that variables set here will take precedence over the same setting in config.json. This includes
## the system console as well and settings set with env variables will be greyed out.

## Below one can find necessary settings to spin up the Mattermost container
MM_SQLSETTINGS_DRIVERNAME=postgres
MM_SQLSETTINGS_DATASOURCE=postgres://${POSTGRES_USER}:${POSTGRES_PASSWORD}@postgres:5432/${POSTGRES_DB}?sslmode=disable&connect_timeout=10

## Example settings (any additional setting added here also needs to be introduced in the docker-compose.yml)
MM_SERVICESETTINGS_SITEURL=https://${DOMAIN}

Any advice? Thanks and best regards.