I worked there for two years (summer 2013-summer 2015), so long enough to get an idea of the culture. Take this post with the caveat that I have no idea whether it's changed since then and if so to what extent.
> Is there any chance I'll be able to work on OSS and/or "modern" tech (e.g. containers, Go, etc.) without a ton of push-back?
Sure there's a greater chance than zero, but not if only motivated by it being "modern". Amazon is a business. It exists to make money by providing a valuable service to customers. The only reason it's a "tech company" is because writing software serves that goal, but "cool tech" is not a goal in itself. If you have a serious, documentable reason why rewriting your team's service in Go would help you achieve business goals, then you might be able to get the attention of engineering decision makers, but that's a much higher bar than "it's modern".
Personally I don't see what is "modern" about Go or how it would help the business serve customers better and/or make more money than it does using Java. I suspect many of your coworkers would feel the same way and these are the terms that the decision would be framed in within Amazon's culture.
By the way there has been over the last several years a move away from C++ and Perl and towards Java. When I was there the majority of stuff was in Java but there was still plenty of important stuff in those other two languages. I suspect C++ and Perl are even rarer now. I guess that's the modernization you're talking about, but maybe not at the pace you want.
> It also seems Amazon is obsessed with reinventing wheels
In many cases, either these wheels were invented at Amazon before they became available in the OSS world, or the OSS tools do not fit Amazon's needs (especially w.r.t. scale).
There is certainly no "obsession" with reinventing wheels -- any sane manager at Amazon would definitely rather use something off-the-shelf than waste a bunch of money developing it from scratch, assuming it fit their needs well.
> and keeping stuff internal
Well, I guess this is true. Amazon contributes less to open source than a few other famous tech companies.
> Is there any chance to introduce solid OSS tools to the development process?
Amazon uses plenty of very solid OSS tools: for example, Linux, Java, gcc, perl, git (though they were on Perforce when I started), and Tomcat are all core parts of Amazon infrastructure. As well as the same grab bag of common tools and libraries you'd find in use anywhere else. In general things with permissive licenses (BSD/MIT/etc) are fair game, but getting approval for things with copyleft licenses (GPL/LGPL/etc) is an uphill battle. As for whether you could introduce more, it would depend on the value of the tool and your motivation for doing so.