Add docker container to generate parsers and languages (#154)

The cucumber/build container is huge. Most people only need it to run
Berp. So, instead of downloading and maintaining a multi-gigabyte
container, lets use a lean one with exactly one purpose.

CONTRIBUTING.md

@@ -24,38 +24,46 @@ localisations and generating the parser is done separately from building each pr
 
 ## Generating parsers
 
-Prerequisites:
+*Either* start a docker container
+
+```shell
+docker build --tag berp-env . 
+docker run --rm --interactive --tty --volume ".:/app" berp-env
+```
+
+*Or* install on your system
 
 * .NET 5.0 (to run `berp` to generate parsers)
 * `berp` (install with `dotnet tool update Berp --version 1.3.0 --tool-path /usr/bin` )
 * `make`
-* `jq` (>= 1.4 for `--sort-keys` option)
 * `diff`
-* `git`
 
-With all this installed use Make:
+With either of these done run:
 
-```
-make generate
+```shell
 make clean-generate
+make generate
 ```
 
 ## Adding or updating an i18n language
 
-Prerequisites:
+1) Edit `gherkin-langauges.json`.
 
-* `make`
-* `jq` (>= 1.4 for `--sort-keys` option)
-* `git`
+2) Distribute the changes to the different parser implementations:
 
-1) Edit `gherkin-langauges.json`.
+    *Either* start a docker container. See [Generating Parsers](#generating-parsers)
 
-2) Distribute the changes to the different parser implementations, this requires `make`, `jq`, `diff`, but no compiler/interpreters:
+    *Or* install on your system
 
-```
-make clean-gherkin-languages
-make copy-gherkin-languages
-```
+   * `make`
+   * `jq` (>= 1.4 for `--sort-keys` option)
+
+    Then with either of these done run
+
+    ```shell
+    make clean-gherkin-languages
+    make copy-gherkin-languages
+    ```
 
 3) Make a pull request with the changed files.
 

Dockerfile

@@ -0,0 +1,25 @@
+# Builds a docker image used for building most projects in this repo. It's
+# used both by contributors and CI.
+#
+FROM mcr.microsoft.com/dotnet/sdk:6.0
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update \
+    && apt-get install --assume-yes \
+    make \
+    jq
+
+
+WORKDIR /app
+
+## Trigger first run experience by running arbitrary cmd to populate local package cache
+RUN dotnet --list-sdks
+
+# Install Berp (dotnet tool installs are user-global; not system global)
+RUN dotnet tool install --global Berp --version 1.4.0 \
+      && echo 'export PATH="$PATH:/root/.dotnet/tools"' >> ~/.bashrc
+
+WORKDIR /app
+
+CMD ["/bin/bash"]