This PR updates the .devcontainer Dockerfile which was no longer
working. Attempting to build an image from the Dockerfile would result
in an error:
```
E: The repository 'http://archive.ubuntu.com/ubuntu kinetic Release' does not have a Release file.
```
AFAIU, this is because the 22.04 release is no longer supported. I
changed the base image to the current LTS release, which should remain
supported until 2029.
In addition, the Dockerfile is also modified to install `emscripten` APT
package. While this is not the recommented installation method from the
Emscripten docs, it is much more convenient and works enough to build
the simulator using the resulting Docker image.
