refactor: update Dockerfile (modern practices, customizable image)

Upgrade implicit image from mysql 5.7 to 8.0, allow customization via
TEST_MYSQL_DOCKER_IMAGE environment variable.

tests/db/Dockerfile

@@ -1,14 +1,13 @@
-FROM mysql:5.7.42-debian
-LABEL maintainer="Seznam.cz a.s."
+ARG TEST_MYSQL_DOCKER_IMAGE
+FROM ${TEST_MYSQL_DOCKER_IMAGE}
+LABEL org.opencontainers.image.authors="Seznam.cz a.s."
 
 WORKDIR /
-ENV MYSQL_ROOT_PASSWORD password
+ENV MYSQL_ROOT_PASSWORD=password
 ADD test_database.sql /docker-entrypoint-initdb.d/
 
 # Tests require us to restart running mysql instance. To this end, we will
 # install and configure supervisord.
-RUN apt-get update || apt-get install -y wget
-RUN wget -O - https://repo.mysql.com/RPM-GPG-KEY-mysql-2023 | gpg --dearmor > /etc/apt/keyrings/mysql.gpg
 RUN \
 apt-get update && apt-get install -y \
     patch \

tests/runtest.sh

@@ -9,7 +9,8 @@ set -e
 PREFIX="`hostname`-`id --user`-`echo $$`-"
 IMAGE_NAME="${PREFIX}superiormysqlpp-test-mysql"
 CONTAINER_NAME="${PREFIX}superiormysqlpp-testdb"
-docker build --pull --tag=${IMAGE_NAME} db
+TEST_MYSQL_DOCKER_IMAGE="${TEST_MYSQL_DOCKER_IMAGE:-mysql:8.0.39-debian}"
+docker build --pull --tag=${IMAGE_NAME} --build-arg=TEST_MYSQL_DOCKER_IMAGE="${TEST_MYSQL_DOCKER_IMAGE}" db
 docker rm --force ${CONTAINER_NAME} >/dev/null 2>&1 || true
 docker run --detach --publish-all --name ${CONTAINER_NAME} ${IMAGE_NAME}
 MYSQL_HOST=`docker inspect --format='{{.NetworkSettings.IPAddress}}' ${CONTAINER_NAME}`