Add dockerfile and more readme instructions

Dockerfile

@@ -0,0 +1,13 @@
+FROM ruby:3.1.2-slim
+
+RUN apt-get update && apt-get install -y \
+    build-essential \
+    libglib2.0-dev \
+    ragel \
+ && rm -rf /var/lib/apt/lists/*
+
+COPY Gemfile Gemfile.lock ./
+RUN gem install bundler:2.3.12 && \
+  bundle install -j$(nproc)
+
+WORKDIR /app

README.md

@@ -5,3 +5,12 @@ Uses the [ragel state machine](https://github.com/adrian-thurston/ragel) to gene
 ## Getting started
 
 Most of the changes will only need to touch `dtext.rl`, the rest of the files will be generated for you by running either `rake compile` or `rake test`. Take a look at [this inofficial quickstart guide](https://github.com/calio/ragel-cheat-sheet) or the [complete official documentation](http://www.colm.net/files/ragel/ragel-guide-6.10.pdf) if you want to know more about how ragel works.
+
+There's a `docker-compose.yml` which you can use to quickly run the most common commands without installing everything locally. Usable like this: `docker-compose run --rm rake test`. You will need to run `docker-compse build` once beforehand.
+
+## Releasing a new version for usage in e621
+
+Commit the changes to `dtext.rl` and the resuling changes in `dtext.c`. Bump the version number in `dtext_rb.gemspec` and `VERSION`. After that is all done you can `bundle lock` in the e6 repository. It should pick up on the increased version.
+
+To test these changes locally commit them and update the `Gemfile`s dtext entry. Specifying the commit hash allows you to rebuild the container without having to also increment the version number every time. Don't forget to `bundle lock` before rebuilding.  
+`gem "dtext_rb", git: "https://github.com/YOUR_FORK/dtext_rb.git", ref: "YOUR_COMMIT_HASH"`

docker-compose.yml

@@ -0,0 +1,16 @@
+version: "3"
+
+services:
+  dtext:
+    build: ./
+    image: dtext
+    volumes:
+      - .:/app
+
+  rake:
+    image: dtext
+    volumes:
+      - .:/app
+    entrypoint: rake
+    profiles: 
+      - "rake"