From what I heard, salt is usually packaged with iodine or some substances that prevent clumping that expire over time. So after some time the salt won't have those anymore, but it should be safe to consume. Salt cannot spoil because bacteria cannot grow in salty places.
Don't know how plastic containers relate to that sadly.
The meaning and ideas of solarpunk are still evolving, but the main themes are freedom, community, ecology and pragmatism. I won't go over the anarchic organisation of communities since I think you mistook the pragmatism for primitivism.
Solarpunk is not about primitivism and a return to a low-technological era, and neither is it a high tech cyberpunk spinoff, as some others think. Solarpunk is about using practical solutions that are also ethical and egolocially friendly. This often means not throwing stuff away, but fixing what can be fixed and reusing what can be reused, because mass production and consumerism is seen as a damaging force. So instead of trying to make up new tech and produce new things, solarpunk would ask you to first consider whether you can do something already with what you have, which means that a DIY approach is encouraged. However, if new technology can improve our lives without damaging everything else, it's acceptable.
And it is the complete opposite of thinking about the "good old days", as solarpunk is looking only towards the future. The 'punk' in the name means that when you look at all the doom and gloom in the future (capitalism, wars, global warming) you don't fall into despair, but instead try to play your part in your community to fight it and promote a lifestyle of mutual aid and a respect for nature, with whatever level of technology can give you the best results.
That was my attempt at a short presentation. We have a wiki and a manifesto if anyone is interested