Change git hooks directory and add README

README.md

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+# docker-ruby
+
+This repository provides Dockerfiles for Alpine and CentOS based images with Ruby. Dockerfiles are generated with pre-commit git hook (`./git-hooks/pre-commit`) and corresponding template in the project's root, so one needs to run `git config core.hooksPath git-hooks` first in order to use it.
+
+Alpine template is based on official [docker-library/ruby](https://github.com/docker-library/ruby).

git-hooks/pre-commit

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+#!/bin/bash
+# Script is used to generate all the necessary Dockerfiles based on the templates
+
+declare -a ruby_versions=("2.5.3" "2.6.2" "2.6.3" "2.6.5")
+declare -a distributions=("alpine:3.12" "centos:7")
+
+for i in "${distributions[@]}"
+do
+  DISTRO_NAME="$(cut -d ':' -f1 <<< $i)"
+  DISTRO_VERSION="$(cut -d ':' -f2 <<< $i)"
+
+  for j in "${ruby_versions[@]}"
+  do
+    # E.g. ./alpine/3.11/2.5.3
+    TARGET_DIR="./generated/$DISTRO_NAME/$DISTRO_VERSION/$j"
+    RUBY_MAJOR="$(echo $j | grep --only-matching -E '[[:digit:]]+\.[[:digit:]]+')"
+
+    mkdir --parents $TARGET_DIR
+    cp ./Dockerfile.$DISTRO_NAME.template $TARGET_DIR/Dockerfile
+
+    sed -i "s/{RUBY_VERSION}/$j/g" $TARGET_DIR/Dockerfile
+    sed -i "s/{RUBY_MAJOR}/$RUBY_MAJOR/g" $TARGET_DIR/Dockerfile
+    sed -i "s/{DISTRO_VERSION}/$DISTRO_VERSION/g" $TARGET_DIR/Dockerfile
+  done
+done
+
+git add ./generated