System (e.g. snap) installed Eclipse on linux does not allow adding j2s plugin to the eclipse/dropins folder

[~mmwarowny] Reports that a system installed Eclipse (e.g. via .deb, .rpm, snap) will not allow a non-privileged user to put the j2s plugin in the application eclipse/dropins directory which is the preferred way to develop JalviewJS or other j2s applications. In fact a snap installed eclipse (which is anecdotally the most stable installation method in e.g. ubuntu) is in a read-only mounted filesystem, so nae chance!

The workaround is to use the command line arguments found in build.gradle in the jalviewjsTranspile task that instruct eclipse to look in a different dir for plugins. Run eclipse as

eclipse --launcher.appendVmargs -vmargs -Dorg.eclipse.equinox.p2.reconciler.dropins.directory=/path/to/external/dropinsdir

which Mateusz has confirmed works.

This information should be put into the developer documetnation for jalviewjs (doc/building.md, or later doc/building_js.md), and could be put into a helper script, e.g. utils/jalviewjs/eclipsej2s.sh containing
#!/usr/bin/env bash
eclipse $@ --launcher.appendVmargs -vmargs -Dorg.eclipse.equinox.p2.reconciler.dropins.directory=./utils/jalviewjs/eclipse/dropins

and a copy of swingjs/net.sf.j2s.core-j11.jar will need to be put in there too. Unfortunately using ./swingjs might be problematic because of the presence of j8 and j11 versions of the plugin, and possibly archived versions too.