Docker: Extensions are always downloaded again

[alexdima]

When using one of the new images, like typescript-node:0-12, and then rebuilding the container, one thing appears to take a lot of time compared to the rest of the steps (2s out of a total of 4s):

[1671 ms] Start: Run in container: cd /root/.vscode-server-insiders/bin/c4b6a6b537de49a734d0eedea06152ea37dfdb6a; export VSCODE_AGENT_FOLDER=/root/.vscode-server-insiders; /root/.vscode-server-insiders/bin/c4b6a6b537de49a734d0eedea06152ea37dfdb6a/server.sh --disable-telemetry --install-extension dbaeumer.vscode-eslint --install-extension ms-vscode.vscode-typescript-tslint-plugin --force
[3538 ms] Installing extensions...
Installing extension 'dbaeumer.vscode-eslint' v2.0.15...
Extension 'dbaeumer.vscode-eslint' v2.0.15 was successfully installed.
Installing extension 'ms-vscode.vscode-typescript-tslint-plugin' v1.2.3...
Extension 'ms-vscode.vscode-typescript-tslint-plugin' v1.2.3 was successfully installed.
[3538 ms] 

Rebuilding the container would be twice as fast if this step would be tweaked to cache things. Perhaps the extension can be downloaded and cached somewhere on the UI side, and then the .vsix files installed via command line inside the container.

[fsackur]

There is a mechanism documented here:
https://code.visualstudio.com/docs/remote/containers-advanced#_avoiding-extension-reinstalls-on-container-rebuild

The constraint is that, if you are running as a non-root user, you need to perform an action in the dockerfile.