Patch the gov.uk tech-docs gems after installing

Dockerfile

@@ -12,3 +12,10 @@ WORKDIR /app
 COPY Gemfile Gemfile.lock ./
 
 RUN bundle install
+
+# patch the installed gems
+# see: https://github.com/alphagov/tech-docs-gem/issues/191
+COPY gem-patches/*.patch /tmp/
+RUN patch /usr/local/bundle/gems/middleman-search-gds-0.11.1/lib/middleman-search/search-index-resource.rb -i /tmp/search-index-resource.rb.patch
+RUN patch /usr/local/bundle/gems/govuk_tech_docs-2.0.12/lib/assets/javascripts/_modules/search.js -i /tmp/search.js.patch
+RUN mv /usr/local/bundle/gems/govuk_tech_docs-2.0.12/lib/assets/javascripts/_modules/search.js /usr/local/bundle/gems/govuk_tech_docs-2.0.12/lib/assets/javascripts/_modules/search.js.erb

gem-patches/README.md

@@ -0,0 +1,32 @@
+# Patches for gov.uk tech-docs gems
+
+These files represent the patches described in this issue:
+https://github.com/alphagov/tech-docs-gem/issues/191
+
+They enable using (a patched version of) the gov.uk tech-docs gem to publish a
+documentation site using github pages without requiring the site to be
+published at the root docpath ("/") of a dedicated domain.
+
+i.e. this enables publishing a site at:
+
+```
+https://username.github.io/my-repo
+```
+
+This is the default github pages URL for the repository "my-repo" belonging to
+the github user/organisation "username".
+
+As opposed to:
+
+```
+https://my.custom.domain/
+```
+
+Without these patches, the search function of the site will not work correctly.
+
+The patch files in this directory are used in the Dockerfile to build the
+docker image which is used in the publishing process.
+
+This is a nasty hack, but the alternative is to maintain forks of two gems,
+`govuk_tech_docs` and `middleman-search-gds`, so this seems a pragmatic
+solution until the issue above is resolved in the upstream gems.

gem-patches/search-index-resource.rb.patch

@@ -0,0 +1,10 @@
++++ /usr/local/bundle/gems/middleman-search-gds-0.11.1/lib/middleman-search/search-index-resource.rb
+@@ -111,6 +111,8 @@
+           end
+         end
+
++        store.each { |id, item| item[:url].sub!(/^/, ENV["ROOT_DOCPATH"].to_s) }
++
+         # Generate JSON output
+         context = ExecJS.compile(source.join("\n"))
+         json = context.eval('lunr.middlemanSearchIndex.indexJson()')

gem-patches/search.js.patch

@@ -0,0 +1,10 @@
++++ /usr/local/bundle/gems/govuk_tech_docs-2.0.12/lib/assets/javascripts/_modules/search.js
+@@ -34,7 +34,7 @@
+     this.downloadSearchIndex = function downloadSearchIndex () {
+       updateTitle('Loading search index')
+       $.ajax({
+-        url: '/search.json',
++        url: '<%= ENV["ROOT_DOCPATH"] %>/search.json',
+         cache: true,
+         method: 'GET',
+         success: function (data) {