Add script for running Maven in Docker

System Lambda cannot be build with newer JDKs, but some developers don't
have JDK 8 installed anymore. The new scripts allows them to build
System Lambda without having JDK 8 as long as they have Docker
installed.

.editorconfig

@@ -9,6 +9,6 @@ insert_final_newline = true
 tab_width = 4
 trim_trailing_whitespace = true
 
-[*.{xml,yml}]
+[*.{sh,xml,yml}]
 indent_size = 2
 indent_style = space

README.md

@@ -358,6 +358,15 @@ System Lambda supports [GitHub Actions](https://help.github.com/en/actions)
 integration. Your pull request will be automatically built by both CI
 servers.
 
+### Build with Docker
+
+The script
+
+    ./scripts/mvn_in_docker.sh clean verify -Dgpg.skip
+
+builds System Lambda inside a Docker container. It uses your Maven local
+repository. This is helpful if you have a Linux or macOS without JDK 8.
+
 
 ## Release Guide
 

pom.xml

@@ -9,7 +9,7 @@
   </parent>
 
   <artifactId>system-lambda</artifactId>
-  <version>1.2.0</version>
+  <version>1.3.0-SNAPSHOT</version>
   <packaging>jar</packaging>
 
   <name>System Lambda</name>

scripts/mvn_in_docker.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+#
+# Executes Maven inside a Docker container with Java 8. The image has to be
+# executed with options, phases and goals just like the original mvn command.
+# E.g. instead of "mvn verify" you have to execute
+#
+#     scripts/mvn_in_docker.sh verify
+#
+# from the project directory. The user's Maven local repository is used by the
+# container.
+
+set -euxo pipefail
+
+readonly group=`id -g`
+readonly user=`id -u`
+
+docker run \
+  --rm \
+  -e MAVEN_CONFIG=/var/maven/.m2 \
+  -u "$user:$group" \
+  -v "$(pwd)":/usr/src/system-lambda \
+  -v ~/.m2:/var/maven/.m2 \
+  -w /usr/src/system-lambda \
+  maven:3.8.1-openjdk-8-slim \
+  mvn -Duser.home=/var/maven "$@"
+rm -r '?/' # I don't know why this directory is created.