> This is annoying and error prone if you use an interpreted language that relies on loading source files at runtime.
That's a bit of misconception. (Not to mention terminology mish-mash) what you probably call "interpreted" is languages like Python, JavaScript or Ruby. In all these cases, the projects are supposed to be compiled into an installable package, and then that package is supposed to be installed. So, compilation step is very similar to languages like C or Go.
Regrettably, developers rarely follow the due process and "deploy" what essentially amounts to the project's source code... This has a bunch of other problems, beside the security issues with potentially copying passwords into the deployment environment. This whole process reminds me of the early days of PHP where Web was full of examples of "guest books" which taught a generation of PHP programmers to interpolate values straight from the URL request into SQL queries. And then, PHP took the blame for "allowing it".
Yeah, this is a bit of a pet peeve of mine. I've seen a lot of Python app Docker images which are built by just copying the git repo straight into the image. Better to build a package from the source in one CI step, and then install that package into the deploy image in another.
Overall, two-phase docker build is a much better model that avoids a lot of miscellaneous issues, including this one. But its also not super well-known for devs who just touch docker every now and then.
For a Go service? Sure, that's easy and makes a ton of sense.