itsdangerous.exc.BadSignature: No b'.' found in value

I have a flask app that I am loading through Docker, and when I try and access the application on localhost:8000 I get the error message in the subject line. I believe the issue is that the flask application is not recognizing my application's SECRET_KEY, but I'm not sure how to fix it.

Here is my app structure (condensed for clarity):

config/
  -- settings.py
instance/
  -- settings.py
myapp/
  -- app.py
  blueprints/
    user/
      -- models.py
.env
docker-compose
Dockerfile

My app-factory function looks like this in app.py:

def create_app(settings_override=None):
    """
    Create a Flask application using the app factory pattern.

    :param settings_override: Override settings
    :return: Flask app
    """
    app = Flask(__name__, instance_relative_config=True)

    app.config.from_object('config.settings')
    app.config.from_pyfile('settings.py', silent=True)

    if settings_override:
        app.config.update(settings_override)

    app.register_blueprint(admin)
    app.register_blueprint(page)
    app.register_blueprint(contact)
    app.register_blueprint(user)
    extensions(app)
    authentication(app, User)

    return app

The error is being triggered in the function called authentication:

def authentication(app, user_model):
    """
    Initialize the Flask-Login extension (mutates the app passed in).

    :param app: Flask application instance
    :param user_model: Model that contains the authentication information
    :type user_model: SQLAlchemy model
    :return: None
    """
    login_manager.login_view = 'user.login'

    @login_manager.user_loader
    def load_user(uid):
        return user_model.query.get(uid)

    @login_manager.token_loader
    def load_token(token):
        duration = app.config['REMEMBER_COOKIE_DURATION'].total_seconds()
        serializer = URLSafeTimedSerializer(app.secret_key)

        data = serializer.loads(token, max_age=duration)
        user_uid = data[0]

        return user_model.query.get(user_uid)

It's the line where it says data = serializer.loads(token, max_age=duration)

The token is usually generated from the secret_key of the application.

Here are some examples from my User class where a token is generated:

def serialize_token(self, expiration=3600):
    """
    Sign and create a token that can be used for things such as resetting
    a password or other tasks that involve a one off token.

    :param expiration: Seconds until it expires, defaults to 1 hour
    :type expiration: int
    :return: JSON
    """
    private_key = current_app.config['SECRET_KEY']

    serializer = TimedJSONWebSignatureSerializer(private_key, expiration)
    return serializer.dumps({'user_email': self.email}).decode('utf-8')

The SECRET_KEY variable is being set from my settings.py file from my config folder. Here is its contents:

from datetime import timedelta

DEBUG = True

SERVER_NAME = 'localhost:8000'
SECRET_KEY = 'insecurekeyfordev'

# Flask-Mail.
MAIL_DEFAULT_SENDER = 'contact@local.host'
MAIL_SERVER = 'smtp.gmail.com'
MAIL_PORT = 587
MAIL_USE_TLS = True
MAIL_USE_SSL = False
MAIL_USERNAME = 'you@gmail.com'
MAIL_PASSWORD = 'awesomepassword'

# Celery.
CELERY_BROKER_URL = 'redis://:devpassword@redis:6379/0'
CELERY_RESULT_BACKEND = 'redis://:devpassword@redis:6379/0'
CELERY_ACCEPT_CONTENT = ['json']
CELERY_TASK_SERIALIZER = 'json'
CELERY_RESULT_SERIALIZER = 'json'
CELERY_REDIS_MAX_CONNECTIONS = 5

# SQLAlchemy.
db_uri = 'postgresql://snakeeyes:devpassword@postgres:5432/snakeeyes'
SQLALCHEMY_DATABASE_URI = db_uri
SQLALCHEMY_TRACK_MODIFICATIONS = False

# User.
SEED_ADMIN_EMAIL = 'dev@local.host'
SEED_ADMIN_PASSWORD = 'devpassword'
REMEMBER_COOKIE_DURATION = timedelta(days=90)

I don't know why this information isn't loading correctly in the app, but when I run docker-compose up --build I get the error message in the title.

If it's at all useful here are the contents of my docker files.

docker-compose.yml

version: '2'

services:
  postgres:
    image: 'postgres:9.5'
    env_file:
      - '.env'
    volumes:
      - 'postgres:/var/lib/postgresql/data'
    ports:
      - '5432:5432'

  redis:
    image: 'redis:3.0-alpine'
    command: redis-server --requirepass devpassword
    volumes:
      - 'redis:/var/lib/redis/data'
    ports:
      - '6379:6379'

  website:
    build: .
    command: >
      gunicorn -b 0.0.0.0:8000
        --access-logfile -
        --reload
        "snakeeyes.app:create_app()"
    env_file:
      - '.env'
    volumes:
      - '.:/snakeeyes'
    ports:
      - '8000:8000'

  celery:
    build: .
    command: celery worker -l info -A snakeeyes.blueprints.contact.tasks
    env_file:
      - '.env'
    volumes:
      - '.:/snakeeyes'

volumes:
  postgres:
  redis:

And my DOCKERFILE:

FROM python:3.7.5-slim-buster
MAINTAINER My Name <myname@gmail.com>

RUN apt-get update && apt-get install -qq -y \
  build-essential libpq-dev --no-install-recommends

ENV INSTALL_PATH /myapp
RUN mkdir -p $INSTALL_PATH

WORKDIR $INSTALL_PATH

COPY requirements.txt requirements.txt
RUN pip install -r requirements.txt

COPY . .
RUN pip install --editable .

CMD gunicorn -b 0.0.0.0:8000 --access-logfile - "myapp.app:create_app()"