Allow for customization of the Jetty WebAppContext

[nscuro]

Standalone Jetty installations provide a means to configure the context via XML files: https://jetty.org/docs/jetty/12/operations-guide/xml/index.html

The mechanism doesn't work for embedded use - XML files need to be discovered and loaded explicitly. For our purposes, applications can place a jetty-context.xml file in their WEB-INF directory in order for Alpine to discover them.

Since Alpine applications do not directly depend on alpine-executable-war, and thus do usually not have access to Jetty classes, the Jetty-native XML configuration was preferred over programmatic customization.

Example jetty-context.xml that disabled classpath scanning:

<?xml version="1.0"?>
<!DOCTYPE Configure PUBLIC "-//Jetty//Configure//EN" "https://jetty.org/configure_10_0.dtd">
<Configure class="org.eclipse.jetty.ee10.webapp.WebAppContext">
    <Call name="setAttribute">
        <Arg>org.eclipse.jetty.server.webapp.ContainerIncludeJarPattern</Arg>
        <Arg>^$</Arg>
    </Call>
    <Call name="setAttribute">
        <Arg>org.eclipse.jetty.server.webapp.WebInfIncludeJarPattern</Arg>
        <Arg>^$</Arg>
    </Call>
</Configure>