Add option to run gradle via docker

.gitignore

@@ -5,4 +5,6 @@ out
 local.properties
 .idea
 .DS_Store
-build
+build
+
+/.gradle-docker-cache

CONTRIBUTING.md

@@ -36,7 +36,7 @@ To contribute code to ODK Validate, you will need to open a [pull request](https
 
 1. Keep your pull request focused on one narrow goal. This could mean addressing an issue with multiple, smaller pull requests. Small pull requests are easier to review and less likely to introduce bugs. If you would like to make stylistic changes to the code, create a separate pull request.
 
-1. Run `./gradlew lint` and `./gradlew check` and fix any errors.
+1. Run `./gradlew lint && ./gradlew check`, or `./gradle-docker lint && ./gradle-docker check`,  and fix any errors.
 
 1. Write clear code. Use descriptive names and create meaningful abstractions (methods, classes).
 

gradle-docker

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+imageTag="gradlew-jdk-8"
+
+log() { echo >&2 "[gradle-docker] $*"; }
+
+rebuildImage=false
+if [[ -n "${REBUILD_GRADLE_DOCKER_IMAGE-}" ]]; then
+  rebuildImage=true
+fi
+
+log "Checking for existing image..."
+if [[ "$rebuildImage" = false ]] && docker image inspect "$imageTag" >/dev/null 2>&1; then
+  log "Existing image found."
+else
+  log "Building docker image..."
+  docker build --tag "$imageTag" --file - . <<EOF
+FROM eclipse-temurin:8-jdk
+WORKDIR /app
+ENV GRADLE_USER_HOME=/app/.gradle-docker-cache
+ENTRYPOINT ["/app/gradlew"]
+EOF
+  log "Docker image built."
+fi
+
+currentUser="$(id -u):$(id -g)" # ensure ownership of docker-created files
+exec docker run --rm \
+    --user "$currentUser" \
+    --volume "$PWD":/app \
+    "$imageTag" \
+    "$@"