Use explicit openjdk:8 Tag

[NColey]

Sbt Native Packager uses openjdk:8 as the default Docker image. This version has been removed from the docker registry, posing issues for projects trying to incorporate sbt-native-packager today.

It also appears to be breaking sbt-native-packagers own builds (see latest commit here) and open PRs (e.g here).

There is an issue in the project opened just under a year ago discussing replacing openjdk:8 with a newer image but as of the latest comment 2 months ago it seems like there is not yet alignment on what to replace openjdk:8 with.

To remove the immediate obstacle while that discussion continues I'd like to request a replacement with the explicit openjdk:8 tag of openjdk:8u342 in line with this suggestion from the docker-library/openjdk repo, full list of tags also mentioned here