fix ref

README.md

@@ -29,7 +29,7 @@ A `devcontainer.json` file is similar to `launch.json` for debugging, but design
 
 ### Why do Dockerfiles in this repo use `RUN` statements with commands separated by `&&`?
 
-Each `RUN` statement creates a Docker image "layer". If one `RUN` statement adds temporary contents, these contents remain in this layer in the image even if they are deleted in a subsequent `RUN`. This means the image takes more storage locally and results in slower image download times if you publish the image to a registry. You can resolve this problem by using a `RUN` statement that includes any clean up steps (separated by `&&`) after a given operation. See [CONTRIBUTING.md](./CONTRIBUTING.md#why-do-dockerfiles-in-this-repository-use-run-statements-with-commands-separated-by-) for more tips.
+Each `RUN` statement creates a Docker image "layer". If one `RUN` statement adds temporary contents, these contents remain in this layer in the image even if they are deleted in a subsequent `RUN`. This means the image takes more storage locally and results in slower image download times if you publish the image to a registry. You can resolve this problem by using a `RUN` statement that includes any clean up steps (separated by `&&`) after a given operation. You can find more tips [here](./docs/TIPS.md/#why-do-dockerfiles-in-this-repository-use-run-statements-with-commands-separated-by).
 
 ### How can I contribute?
 

docs/TIPS.md

@@ -0,0 +1,43 @@
+### Why do Dockerfiles in this repository use RUN statements with commands separated by &&?
+
+Each `RUN` statement creates a Docker image "layer". If one `RUN` statement adds in temporary contents, these contents remain in this layer in the image even if they are deleted in a subsequent `RUN`. This means the image takes more storage locally and results in slower image download times if you publish the image to a registry.
+
+So, in short, you want to clean up after you install or configure anything in the same `RUN` statement. To do this, you can either:
+
+1. Use a string of commands that cleans up at the end. e.g.: 
+
+    ```Dockerfile
+    RUN apt-get update && apt-get -y install --no-install-recommends git && apt-get autoremove -y && apt-get clean -y && rm -rf /var/lib/apt/lists/*
+    ```
+
+    ... or across multiple lines (note the `\` at the end escaping the newline):
+
+    ```Dockerfile
+    RUN apt-get update \
+        && apt-get -y install git \
+        && apt-get autoremove -y \
+        && apt-get clean -y \
+        && rm -rf /var/lib/apt/lists/*
+    ```
+
+2. Put the commands in a script, temporarily copy it into the container, then remove it. e.g.:
+
+    ```Dockerfile
+    COPY ./my-script.sh /tmp/my-script.sh
+    RUN bash /tmp/my-script.sh \
+        && rm -f /tmp/my-script.sh
+    ```
+
+Some other tips:
+
+1. You'd be surprised [how big package lists](https://askubuntu.com/questions/179955/var-lib-apt-lists-is-huge) can get, so be sure to clean these up too. Most Docker images that use Debian / Ubuntu use the following command to clean these up:
+
+    ```
+    rm -rf /var/lib/apt/lists/*
+    ```
+
+The only downside of doing this is that `apt-get update` has to be executed before you install a package. However, in most cases adding this package to a Dockerfile is a better choice anyway since this will survive a "rebuild" of the image and the creation of an updated container. 
+
+2. In all cases, you'll want to pay attention to package caching since this can also take up image space. Typically there is an option for a package manager to not cache when installing that you can use to minimize the size of the image. For example, for Alpine Linux, there's `apk --no-cache`
+
+3. Watch out for the installation of "recommended" packages you don't need. By default, Debian / Ubuntu's `apt-get` installs packages that are commonly used with the one you specified - which in many cases isn't required. You can use `apt-get -y install --no-install-recommends` to avoid this problem.