Completely update everything based on the few things I've learned since

.gitignore

@@ -1,3 +1 @@
-.coverage
 .env
-htmlcov

Dockerfile

@@ -1,14 +1,25 @@
-FROM python:3.6-slim
+FROM python:3.8-alpine
 
+# Set environment variables
 ENV PYTHONDONTWRITEBYTECODE 1
+ENV PYTHONFAULTHANDLER 1
 ENV PYTHONUNBUFFERED 1
 
-WORKDIR /app
+RUN apk update && apk add --no-cache \
+    # Required for installing/upgrading postgres, Pillow, etc:
+    gcc python3-dev \
+    # Required for installing/upgrading postgres:
+    postgresql-libs postgresql-dev musl-dev
 
-# Uncomment this when https://github.com/pypa/pipenv/issues/2924 is resolved:
-#RUN pip install --upgrade pip
-RUN pip install pipenv
-COPY Pipfile /app/
-RUN pipenv install --skip-lock --system --dev
+# Set work directory
+RUN mkdir /code
+WORKDIR /code
 
-COPY . /app/
+# Install dependencies into a virtualenv
+RUN pip install --upgrade pipenv
+COPY ./Pipfile .
+COPY ./Pipfile.lock .
+RUN pipenv install --dev --deploy
+
+# Copy project code
+COPY . /code/

Pipfile

@@ -4,14 +4,12 @@ verify_ssl = true
 name = "pypi"
 
 [dev-packages]
-django-debug-toolbar = "==1.10.1"
-coverage = "==4.5.1"
+# Any packages required only for development should go here.
 
 [packages]
-dj-database-url = "==0.5.0"
-Django = "==2.1.2"
-gunicorn = "==19.9.0"
-psycopg2 = "==2.7.5"
+Django = ">=3.1, <3.2"
+gunicorn = ">=20, <21"
+psycopg2 = ">=2, <3"
 
 [requires]
-python_version = "3.6"
+python_version = "3.8"