Add warning in v3 about upcoming Docker removal in v4

Dockerfile

@@ -1,6 +1,6 @@
 FROM sonarsource/sonar-scanner-cli:11.1
 
-LABEL version="3.0.0" \
+LABEL version="3.1.0" \
       repository="https://github.com/sonarsource/sonarqube-scan-action" \
       homepage="https://github.com/sonarsource/sonarqube-scan-action" \
       maintainer="SonarSource" \

entrypoint.sh

@@ -2,6 +2,8 @@
 
 set -eo pipefail
 
+echo "::warning title=Docker removed in the next major version::Users on the master branch of this GitHub action will be upgraded automatically on December 9th to its next major version, which replaces Docker with a composite action, executing in the runner environment. Self-hosted runners analyzing JS/TS code against SonarQube 10.2 and below will need to have Node JS installed."
+
 declare -a args=()
 
 if [[ -z "${SONAR_TOKEN}" ]]; then