Handle exclusions when ignoring files

Always exclude Dockerfile and .dockerignore from being ignored.

https://docs.docker.com/engine/reference/builder/#dockerignore-file
> You can even use the .dockerignore file to exclude the Dockerfile and .dockerignore files.
> These files are still sent to the daemon because it needs them to do its job.
> But the ADD and COPY instructions do not copy them to the image.

resolves upserve#484

lib/docker/util.rb

@@ -260,7 +260,7 @@ def build_config_header(credentials)
   end
 
   def glob_all_files(pattern)
-    Dir.glob(pattern, File::FNM_DOTMATCH) - ['..', '.']
+    Dir.glob(pattern, File::FNM_DOTMATCH)
   end
 
   def remove_ignored_files!(directory, files)
@@ -269,11 +269,23 @@ def remove_ignored_files!(directory, files)
     ignored_files(directory, ignore).each { |f| files.delete(f) }
   end
 
-  def ignored_files(directory, ignore_file)
-    patterns = File.read(ignore_file).split("\n").each(&:strip!)
-    patterns.reject! { |p| p.empty? || p.start_with?('#') }
+  def resolve_patterns(patterns, directory)
     patterns.map! { |p| File.join(directory, p) }
     patterns.map! { |p| File.directory?(p) ? "#{p}/**/*" : p }
     patterns.flat_map { |p| p =~ GLOB_WILDCARDS ? glob_all_files(p) : p }
   end
+
+  def ignored_files(directory, ignore_file)
+    patterns = File.read(ignore_file).split("\n").each(&:strip!)
+    patterns.reject! { |p| p.empty? || p.start_with?('#') }
+
+    exclusion_patterns = patterns.select { |p| p.start_with?('!') }
+    inclusion_patterns = patterns - exclusion_patterns
+    exclusion_patterns.map! { |p| p[1..-1] }.concat(%w(Dockerfile .dockerignore))
+
+    files_ignored = resolve_patterns(inclusion_patterns, directory)
+    files_excluded = resolve_patterns(exclusion_patterns, directory)
+
+    files_ignored - files_excluded - [File.join(directory, '.'), File.join(directory, '..')]
+  end
 end

spec/docker/util_spec.rb

@@ -99,6 +99,31 @@
         expect(files_in_tar(tar)).to eq ['.dockerignore', 'baz']
       end
 
+      it 'does not ignore files starting with !' do
+        File.write("#{tmpdir}/Dockerfile", 'bar')
+
+        File.write("#{tmpdir}/.dockerignore", '*')
+
+        expect(files_in_tar(tar)).to eq ['.dockerignore', 'Dockerfile']
+      end
+
+      it 'does not ignore the Dockerfile' do
+        File.write("#{tmpdir}/Dockerfile", 'bar')
+
+        File.write("#{tmpdir}/.dockerignore", '*')
+
+        expect(files_in_tar(tar)).to eq ['.dockerignore', 'Dockerfile']
+      end
+
+      it 'ignores files' do
+        File.write("#{tmpdir}/foo", 'bar')
+        File.write("#{tmpdir}/baz", 'bar')
+
+        File.write("#{tmpdir}/.dockerignore", "foo")
+
+        expect(files_in_tar(tar)).to eq ['.dockerignore', 'baz']
+      end
+
       it 'ignores folders' do
         FileUtils.mkdir("#{tmpdir}/foo")
         File.write("#{tmpdir}/foo/bar", 'bar')
@@ -249,6 +274,61 @@
     end
   end
 
+  describe '.ignored_files' do
+    attr_accessor :tmpdir
+
+    subject do
+      described_class.ignored_files(tmpdir, ignore_file).map do |f|
+        f[tmpdir.length+1..-1]
+      end
+    end
+
+    let(:ignore_file) { "#{tmpdir}/.dockerignore" }
+
+    around do |example|
+      Dir.mktmpdir do |tmpdir|
+        self.tmpdir = tmpdir
+        example.call
+      end
+    end
+
+    it 'ignores explicit files' do
+      File.write("#{tmpdir}/foo", 'bar')
+      File.write("#{tmpdir}/baz", 'bar')
+
+      File.write("#{tmpdir}/.dockerignore", "foo")
+
+      expect(subject).to eq %w(foo)
+    end
+
+    it 'makes an exception to exclusions' do
+      File.write("#{tmpdir}/foo", 'bar')
+      File.write("#{tmpdir}/baz", 'bar')
+
+      File.write("#{tmpdir}/.dockerignore", "foo\n!foo")
+
+      expect(subject).to be_empty
+    end
+
+    it 'makes an exception to exclusions (order doesnt matter)' do
+      File.write("#{tmpdir}/foo", 'bar')
+      File.write("#{tmpdir}/baz", 'bar')
+
+      File.write("#{tmpdir}/.dockerignore", "!foo\nfoo")
+
+      expect(subject).to be_empty
+    end
+
+    it 'does not ignore the Dockerfile or .dockerignore' do
+      File.write("#{tmpdir}/Dockerfile", 'bar')
+      File.write("#{tmpdir}/baz", 'bar')
+
+      File.write("#{tmpdir}/.dockerignore", '*')
+
+      expect(subject).to eq %w(baz)
+    end
+  end
+
   def files_in_tar(tar)
     Gem::Package::TarReader.new(tar) { |content| return content.map(&:full_name).sort }
   end