A stopped or mid-transient (restarting / rebuilding) container's
web server isn't answering, so its <url>/icon background-image
just failed to an empty box on the card.
When the container isn't reachable (not running, or a transient
is in flight) the icon now falls back to the dimmed hyperhive
mark — /favicon.svg, served by the dashboard itself so it's
always loadable — greyscaled + lowered opacity via the
.icon-unreachable class.
closes#195
16f614f45d