README.md: docker instructions to be less dependent on host

README.md

@@ -1,8 +1,9 @@
 # XCSoar.org - Website
 
-1. Install dependencies
+1. Run Ruby Docker image
    ```
-   sudo apt-get install bundler yui-compressor librsvg2-bin build-essential ruby-dev
+   docker run -v ./:/data -it ruby:3.3.5  /bin/bash
+   cd /data
    ```
 
 2. Install ruby dependencies
@@ -19,7 +20,9 @@
 
 1. Generate a stub for the new post
    ```
-   bundle exec rake new_post["XCSoar 6.8.12 released"]
+   git config --global --add safe.directory /data
+   apt update && apt install -y vim
+   bundle exec rake new_post["XCSoar 7.55 released"]
    ```
 
 2. Edit the new post, created in `./_posts`