Container images should be built by makefiles, and mostly on amd64 platform

[cardil]

As discussed in #11 (comment) the build could be enhanced.

Now we are building both frontend and backend for every platform we support. This takes a long time, especially on emulated platforms.

In theory, we could just build it once (on linux/amd64), and then just create a simple multi-arch image from the build artifacts. Both JVM, and TS/JS support the "build once, run everywhere" feature.

Unfortunately, it is not possible with this approach of using FROM ... as frontend/builder, and COPY --from=builder ... because the container engine errors saying it doesn't have an image for given architecture.

Maybe we can achieve that by utilizing volumes, for passing down the artifacts. In that way, we can build frontend, and backend apps once and then just create a multi-arch container image.

Also, as pointed out by @mgencur #11 (comment) it would be a good idea to be able to build them from makefiles, to not over rely on GH actions, and also make it easier to develop.