bump required versions based on Debian Stable

Dockerfile

@@ -1,4 +1,4 @@
-FROM debian:testing-slim
+FROM debian:stable-slim
 ENV DEBIAN_FRONTEND=noninteractive
 EXPOSE 8000
 RUN apt update && apt install -y curl

README.markdown

@@ -23,8 +23,11 @@ check out one of the versioned branches.
 
 # Requirements
 
-- MySQL/MariaDB 5.1+ (with experimental support for PostgreSQL 9+ and SQLite 3)
-- [Stable PHP](https://en.wikipedia.org/wiki/PHP#Release_history) (7.1+ as of writing)
+- These are generally based on "whatever is in Debian Stable", because that's
+  conservative without being TOO painfully out of date, and is a nice target
+  for the unit test Docker build.
+- A database: PostgreSQL 11+ / MariaDB 10.3+ / SQLite 3.27+
+- [Stable PHP](https://en.wikipedia.org/wiki/PHP#Release_history) (7.3+ as of writing)
 - GD or ImageMagick
 
 # Installation

core/sys_config.php

@@ -38,11 +38,11 @@ _d("COMPILE_ELS", false);    // boolean  pre-build the list of event listeners
 _d("NICE_URLS", false);      // boolean  force niceurl mode
 _d("SEARCH_ACCEL", false);   // boolean  use search accelerator
 _d("WH_SPLITS", 1);          // int      how many levels of subfolders to put in the warehouse
-_d("VERSION", '2.7.0');      // string   shimmie version
+_d("VERSION", '2.8-dev');    // string   shimmie version
 _d("TIMEZONE", null);        // string   timezone
 _d("EXTRA_EXTS", "");        // string   optional extra extensions
 _d("BASE_URL", null);        // string   force a specific base URL (default is auto-detect)
-_d("MIN_PHP_VERSION", '7.1');// string   minimum supported PHP version
+_d("MIN_PHP_VERSION", '7.3');// string   minimum supported PHP version
 _d("TRACE_FILE", null);      // string   file to log performance data into
 _d("TRACE_THRESHOLD", 0.0);  // float    log pages which take more time than this many seconds
 _d("ENABLED_MODS", "imageboard");