fix: install pkg-config dependency for mysqlclient>=2.2.0

Repositiories that depend on mysqlclient>=2.2.0 will need to install the package pkg-config in their Dockerfile: PyMySQL/mysqlclient#620. This commit installs the pkg-config package in the Dockerfile.

If this is missing, then pip install of mysqlclient fails with an error that includes the following:

Exception: Can not find valid pkg-config name.
Specify MYSQLCLIENT_CFLAGS and MYSQLCLIENT_LDFLAGS env vars manually

See edx/edx-arch-experiments#349.

Dockerfile

@@ -2,13 +2,18 @@ FROM ubuntu:focal as app
 
 ENV DEBIAN_FRONTEND noninteractive
 
+# Packages installed:
+
+# pkg-config; mysqlclient>=2.2.0 requires pkg-config (https://github.com/PyMySQL/mysqlclient/issues/620)
+
 RUN apt-get update && apt-get install --no-install-recommends -qy \
   language-pack-en \
   build-essential \
   python3.8-dev \
   python3-virtualenv \
   python3.8-distutils \
   libmysqlclient-dev \
+  pkg-config \
   libssl-dev \
   # needed by phantomjs
   libfontconfig \