Dockerfile fix for python-ldap

Dockerfile

-FROM python:3.8-slim
+FROM python:3.7-slim
+
+# requirements to build python-ldap
+RUN apt-get update && apt-get install -y \
+    build-essential python3-dev libldap2-dev libsasl2-dev \
+    && rm -rf /var/lib/apt/lists/*
 
 RUN mkdir /app
 WORKDIR /app