Improvements for Agent app and Docker image

[Zelldon]

There were some improvements I would've liked to see, but it made the PR considerably bigger: adding the EZE version and Zeebe version when logging startup, being able to configure the bind address of the server as well as its transport (so, for example, using Unix domain socket instead of an IP address to completely avoid port collisions), producing a standalone executable JAR, adding a QA module to actually test the image, etc. Would be cool to have one day. Additionally, since this embeds eze, it uses the module's log4j2 dependency as SLF4J implementation - not sure that's the right thing, but probably fine for now.

Originally posted by @npepinpe in #91 (comment)