diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 28e427d756..fce3b90c38 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -28,9 +28,9 @@ To run Linguist from the cloned repository:
 
 ### Dependencies
 
-Linguist uses the [`charlock_holmes`](https://github.com/brianmario/charlock_holmes) character encoding detection library which in turn uses [ICU](http://site.icu-project.org/), and the libgit2 bindings for Ruby provided by [`rugged`](https://github.com/libgit2/rugged). These components have their own dependencies - `icu4c`, and `cmake` and `pkg-config` respectively - which you may need to install before you can install Linguist.
+Linguist uses the [`charlock_holmes`](https://github.com/brianmario/charlock_holmes) character encoding detection library which in turn uses [ICU](http://site.icu-project.org/), and the libgit2 bindings for Ruby provided by [`rugged`](https://github.com/libgit2/rugged). [Docker](https://www.docker.com/) is also required when adding or updating grammars. These components have their own dependencies - `icu4c`, and `cmake` and `pkg-config` respectively - which you may need to install before you can install Linguist.
 
-For example, on macOS with [Homebrew](http://brew.sh/): `brew install cmake pkg-config icu4c` and on Ubuntu: `apt-get install cmake pkg-config libicu-dev`.
+For example, on macOS with [Homebrew](http://brew.sh/): `brew install cmake pkg-config icu4c docker` and on Ubuntu: `apt-get install cmake pkg-config libicu-dev docker-ce`.
 
 ## Adding an extension to a language
 
diff --git a/script/add-grammar b/script/add-grammar
index 1b4f71d008..cdaa3066a5 100755
--- a/script/add-grammar
+++ b/script/add-grammar
@@ -84,6 +84,10 @@ unless $url
   exit 1;
 end
 
+# Exit early if docker isn't installed or running.
+log "Checking docker is installed and running"
+command('docker', 'ps')
+
 # Ensure the given URL is an HTTPS link
 parts    = parse_url $url
 https    = "https://#{parts[:host]}/#{parts[:user]}/#{parts[:repo]}"
@@ -107,6 +111,7 @@ log "Confirming license"
 if repo_old
   command('script/licensed')
 else
+  repo_new = File.absolute_path(repo_new)
   command('script/licensed', '--module', repo_new)
 end