Add example guide for pre-installing vscode extensions with docker installation

Author: axonasif

README.md

@@ -44,6 +44,39 @@ _Note_: Feel free to use the `nightly` tag to test the latest version, i.e. `git
 	```
 - For additional possibilities, please consult the `Dockerfile` for OpenVSCode Server at https://github.com/gitpod-io/openvscode-releases/
 
+#### Pre-installing VSCode extensions
+
+You can pre-install vscode extensions in such a way:
+
+```dockerfile
+FROM gitpod/openvscode-server:latest
+
+ENV OPENVSCODE_SERVER_ROOT="/home/.openvscode-server"
+ENV OPENVSCODE="${OPENVSCODE_SERVER_ROOT}/bin/openvscode-server"
+
+SHELL ["/bin/bash", "-c"]
+RUN \
+    # Direct download links to external .vsix not available on https://open-vsx.org/
+    # The two links here are just used as example, they are actually available on https://open-vsx.org/
+    urls=(\
+        https://github.com/rust-lang/rust-analyzer/releases/download/2022-12-26/rust-analyzer-linux-x64.vsix \
+        https://github.com/VSCodeVim/Vim/releases/download/v1.24.3/vim-1.24.3.vsix \
+    )\
+    # Create a tmp dir for downloading
+    && tdir=/tmp/exts && mkdir -p "${tdir}" && cd "${tdir}" \
+    # Download via wget from $urls array.
+    && wget "${urls[@]}" && \
+    # List the extensions in this array
+    exts=(\
+        # From https://open-vsx.org/ registry directly
+        gitpod.gitpod-theme \
+        # From filesystem, .vsix that we downloaded (using bash wildcard '*')
+        "${tdir}"/* \
+    )\
+    # Install the $exts
+    && for ext in "${exts[@]}"; do ${OPENVSCODE} --install-extension "${ext}"; done
+```
+
 ### Linux
 
 - [Download the latest release](https://github.com/gitpod-io/openvscode-server/releases/latest)