Having defined on my web.xml the JNDI context to locate my own logback configuration, logback is not able to locate the configuration file on WEB-INF/etc directory. As a workaround, I dropped the logback.xml file under WEB-INF/classes directory.

<env-entry>
<env-entry-name>logback/context-name</env-entry-name>
<env-entry-type>java.lang.String</env-entry-type>
<env-entry-value>myapp</env-entry-value>
</env-entry>

<env-entry>
<description>URL for configuring logback context</description>
<env-entry-name>logback/configuration-resource</env-entry-name>
<env-entry-type>java.lang.String</env-entry-type>
<env-entry-value>../etc/logback-myapp.xml</env-entry-value>
</env-entry>