Simplify docker build

Dockerfile

@@ -10,4 +10,8 @@ RUN apt-get update -y \
     woff-tools \
     woff2
 
+WORKDIR /workspace
+COPY Gemfile /workspace
+COPY Gemfile.lock /workspace
+RUN bundle install
 CMD bash

README.md

@@ -100,13 +100,11 @@ Then, run the Docker container:
 
     $ docker run --rm -it \
         -u $(id -u):$(id -g) \
-        -v $(pwd):$(pwd) \
-        -w $(pwd) \
+        -v $(pwd):/workspace \
         fa-builder
 
 Within the container, build the font:
 
-    $ bundle install --path vendor/bundle
     $ npm ci
     $ make -C src/icons