ci: add Dockerfile linting

.travis.yml

@@ -6,5 +6,8 @@ rvm:
 - 2.6
 - 2.5
 
+services:
+- docker
+
 script:
 - bundle exec rake travis

Rakefile

@@ -9,16 +9,23 @@ RSpec::Core::RakeTask.new(:spec)
 RuboCop::RakeTask.new
 Bundler::Audit::Task.new
 
-desc 'Should be run by developer once to prepare initial solargraph usage (fill caches etc.)'
-task :'solargraph:init' do
-  sh 'solargraph download-core'
-end
-
 desc 'Run experimental solargraph type checker'
-task :'solargraph:tc' do
+task :solargraph do
   sh 'solargraph typecheck'
 end
 
-task default: [:rubocop, :spec, 'bundle:audit']
+namespace :solargraph do
+  desc 'Should be run by developer once to prepare initial solargraph usage (fill caches etc.)'
+  task :init do
+    sh 'solargraph download-core'
+  end
+end
 
-task travis: [:default, :'solargraph:init', :'solargraph:tc']
+desc 'Run hadolint for Dockerfile linting'
+task :hadolint do
+  sh 'docker run --rm -i hadolint/hadolint:v1.18.0 hadolint --ignore DL3018 - < docker/Dockerfile'
+end
+
+task default: [:rubocop, :spec, 'bundle:audit', :solargraph]
+
+task travis: ['solargraph:init', :default, :hadolint]