Fix legacy PHP 5.6 Docker build on current Docker/ARM hosts

Dockerfile

@@ -4,11 +4,17 @@ FROM php:5.6-apache
 # Install the mysqli extension
 RUN docker-php-ext-install mysqli
 
-# Update repo
-RUN apt-get update -y
-
-# Install mysql-client
-RUN apt-get install mysql-client -y
+# Update package sources for the archived Debian Stretch base image and
+# install the CLI tools needed during the image build.
+RUN set -eux; \
+  printf '%s\n' \
+    'deb [trusted=yes] http://archive.debian.org/debian stretch main' \
+    'deb [trusted=yes] http://archive.debian.org/debian-security stretch/updates main' \
+    > /etc/apt/sources.list; \
+  printf 'Acquire::Check-Valid-Until "false";\n' > /etc/apt/apt.conf.d/99archive; \
+  apt-get update -o Acquire::Check-Valid-Until=false; \
+  apt-get install -y --no-install-recommends curl default-mysql-client; \
+  rm -rf /var/lib/apt/lists/*
 
 # Install wp-cli as wp
 RUN curl -O https://raw.githubusercontent.com/wp-cli/builds/gh-pages/phar/wp-cli.phar  && \

docker-compose.yml

@@ -1,8 +1,8 @@
-version: '3.1'
-
 services:
 
     db:
+        # MySQL 5.7 is pulled as amd64 on Apple Silicon, so make that explicit.
+        platform: linux/amd64
         # mysql/mysql-server is less in size than mysql
         image: mysql/mysql-server:5.7
         restart: always