Tried ./gradlew clean build on the following docker images: openjdk:8-jdk-slim, java:8 and atlassian/default-image:latest (java-8-openjdk-amd64), and the same on my local machine (jdk1.8.0_162). Cleared the workspace between switching the different Forge versions and reran setup steps described in the README. NoSuchMethodError persisted in launcher until I moved the mappings to stable_39.