Improve build performance

[kroese]

Description

This change improves the buildtime of the Docker container massively, by executing the build stage only once, instead of multiple times for each platform.

Currently when building the ARM64 image it is executing the build-stage under QEMU emulation, but after this change it re-uses the files build for AMD64 (since they are all javascript that does not matter).

[kroese]

@SleeplessOne1917

Sorry, but it does not work:

Node.js v20.10.0
/app/node_modules/sharp/lib/sharp.js:37
  throw new Error(help.join('\n'));
  ^
Error: 
Something went wrong installing the "sharp" module
Cannot find module '../build/Release/sharp-linuxmusl-arm64v8.node'
Require stack:
- /app/node_modules/sharp/lib/sharp.js
- /app/node_modules/sharp/lib/constructor.js
- /app/node_modules/sharp/lib/index.js
- /app/dist/js/server.js
Possible solutions:
- Install with verbose logging and look for errors: "npm install --ignore-scripts=false --foreground-scripts --verbose sharp"
- Install for the current linuxmusl-arm64v8 runtime: "npm install --platform=linuxmusl --arch=arm64v8 sharp"
- Consult the installation documentation: https://sharp.pixelplumbing.com/install
    at Object.<anonymous> (/app/node_modules/sharp/lib/sharp.js:37:9)
    at Module._compile (node:internal/modules/cjs/loader:1376:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1435:10)
    at Module.load (node:internal/modules/cjs/loader:1207:32)
    at Module._load (node:internal/modules/cjs/loader:1023:12)
    at Module.require (node:internal/modules/cjs/loader:1235:19)
    at require (node:internal/modules/helpers:176:18)
    at Object.<anonymous> (/app/node_modules/sharp/lib/constructor.js:11:1)
    at Module._compile (node:internal/modules/cjs/loader:1376:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1435:10)

So this commit has to be reverted unfortunately. I am still convinced it should be possible to cross-compile in the builder-stage, but it will require some more changes than just the one line in this commit.

[kroese]

According to https://sharp.pixelplumbing.com/install#cross-platform it can be fixed by installing both packages at the same time:

npm install --cpu=x64 --os=linux sharp
npm install --cpu=arm64 --os=linux sharp

There might be other packages that Lemmy depends on that contain binaries (I just assumed node packages only contained Javascript). So the final fix would look something like this:

npm install --cpu=$TARGET_PLATFORM

But until we figure this out this pullrequest needs to be reversed. I thought I tested it, but I got tricked because Cloudflare was serving a cached response so it looked like the image was functioning correctly. I should have checked the container log.