Add Dockerfile (#4687)

* added docker image

* added breakdown example to README.md, removed useless packages from docker, fix .dockerignore

* delete useless libs after build

* remove cmake after build

* Add test for dockerfile

* Checkout code

Co-authored-by: Zaytsev Dmitriy <zaytsev_dmitriy@edadeal.ru>
Co-authored-by: Colin Seymour <colin@github.com>

.dockerignore

@@ -0,0 +1,2 @@
+.git/
+test/

.github/workflows/ci.yml

@@ -27,3 +27,16 @@ jobs:
       run: bundle exec rake
     - name: Check Licenses
       run: bundle exec licensed status -c vendor/licenses/config.yml
+  dockerfile:
+    name: Test Dockerfile
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - name: Build container
+      run: docker build -t linguist .
+    - name: Analyse current directory
+      run: docker run --rm -v $(pwd):$(pwd) -w $(pwd) -t linguist > output
+    - name: Confirm analysis
+      run: |
+        cat output
+        grep Ruby output

Dockerfile

@@ -0,0 +1,12 @@
+FROM ruby:alpine
+
+RUN apk --update add --virtual build_deps \
+    build-base \
+    libc-dev \
+    linux-headers \
+    cmake \
+    && apk --no-cache add icu-dev libressl-dev \
+    && gem install github-linguist \
+    && apk del build_deps build-base libc-dev linux-headers cmake
+
+CMD ["github-linguist"]

README.md

@@ -137,6 +137,36 @@ grammars.yml: 884 lines (884 sloc)
   language:  YAML
 ```
 
+#### Docker
+
+If you have Docker installed you can build an image and run Linguist within a container:
+
+```console
+$ docker build -t linguist .
+$ docker run --rm -v $(pwd):$(pwd) -w $(pwd) -t linguist
+68.57%  Ruby
+22.90%  C
+6.93%   Go
+1.21%   Lex
+0.39%   Shell
+$ docker run --rm -v $(pwd):$(pwd) -w $(pwd) -t linguist github-linguist --breakdown
+68.57%  Ruby
+22.90%  C
+6.93%   Go
+1.21%   Lex
+0.39%   Shell
+
+Ruby:
+Gemfile
+Rakefile
+bin/git-linguist
+bin/github-linguist
+ext/linguist/extconf.rb
+github-linguist.gemspec
+lib/linguist.rb
+…
+```
+
 ## Troubleshooting
 
 ### My repository is detected as the wrong language