Devcontainer description in Contributing missing make io command

[jpvolkmann]

After cloning the current master into a VS Code devcontainer according to

https://github.com/evcc-io/evcc/blob/master/CONTRIBUTING.md#debugging-in-vs-code

and trying to start it I got the following error message:

Build Error: go build -o /workspaces/evcc/__debug_bin2615972376 -gcflags all=-N -l .
main.go:15:13: pattern dist: no matching files found (exit status 1)

After following #5878 and executing

make ui

on the docker container command line, EVCC in devcontainer could start.

=> Either devcontainer or documentation should be updated

I'm using MacOS on Apple M1 with "Docker Desktop"