Add command for packing the ZODB

[davisagli]

Packing is done with different commands depending on whether ZEO or Relstorage is used. For Relstorage it needs a modified conf file that only includes the relstorage section. It would be nice for the image to include the necessary configuration and an entrypoint which runs the right command.

(Or are there good reasons to create a separate image for this?)

[Rudd-O]

I support your thesis that same image should include maintenance commands.