add docker compose to simplify running tests

Dockerfile

@@ -0,0 +1,7 @@
+FROM ruby:3.4
+
+WORKDIR /usr/src/app
+
+ARG DEBIAN_FRONTEND=noninteractive
+
+RUN apt update && apt install -y default-jre

README.md

@@ -9,6 +9,19 @@ See tests for examples.
 1. The library, for ZUGFeRD 2.x, currently only supports the EN16931 variant. This is probably what you want as well. PRs welcome.
 3. This does not contain any code to attach the XML to a PDF file, mainly because I have yet to find a ruby library to do that. For software that does this, take a look at [this python library](https://github.com/akretion/factur-x) or [this Java library which also does extended validation](https://mustangproject.org)
 
+## Gem development & test
+### With docker compose installed
+```shell
+docker compose up
+```
+### Without docker compose installed
+* install ruby
+* install jre
+* bundle install
+```shell
+bundle exec rake
+```
+
 ## Contributors
 
 See [CONTRIBUTORS.md](CONTRIBUTORS.md).

docker-compose.yml

@@ -0,0 +1,13 @@
+volumes:
+  bundle:
+
+services:
+  app:
+    build:
+      context: .
+    command: bash -c 'bundle && bundle exec rake'
+    volumes:
+      - .:/usr/src/app
+      - "bundle:/bundle"
+    environment:
+      BUNDLE_PATH: /bundle