fix(docker): pin uWebSockets/uSockets tags so build works on Apple Silicon

[mudassaralichouhan]

Problem

docker compose build search-engine fails on Apple Silicon (M1/M2/M3 Macs)
when running with DOCKER_DEFAULT_PLATFORM=linux/amd64:

make: *** [GNUmakefile:7: examples] Error 255
write jobserver: Bad file descriptor
failed to solve: process "/bin/sh -c make -j$(nproc) WITH_EXAMPLES=0"
    did not complete successfully: exit code: 2

It also makes the build non-deterministic on any host — every rebuild
clones whatever HEAD of uWebSockets / uSockets happens to be on
master that day.

Root cause — two issues stacked

1. Cloning master is unstable. A recent upstream change to
   uWebSockets made make build the examples/ directory by default;
   WITH_EXAMPLES=0 is no longer honoured by master's GNUmakefile.
   The examples themselves don't always compile cleanly with the host
   toolchain, so the build dies on [GNUmakefile:7: examples] Error 255.

2. GNU make's jobserver is buggy under QEMU x86_64 emulation on ARM.
   -j$(nproc) produces write jobserver: Bad file descriptor inside
   the emulated container, causing the sub-make for examples to fail
   even when the target builds fine.

Fix

• Pin to stable tags. uSockets v0.8.6 and uWebSockets v20.74.0
  are the last releases whose Makefiles still respect WITH_EXAMPLES=0.
• -j1 on the uWebSockets step only. Sidesteps the QEMU jobserver
  bug. Adds a few seconds on native amd64; unlocks ARM hosts entirely.
• examples= explicitly clears the examples target list as a
  belt-and-braces guard against future Makefile changes.
• Build ARGs so the tags can be overridden at build time without
  editing the Dockerfile:

  docker build --build-arg UWEBSOCKETS_TAG=v20.77.0 .

Also adds a tracked .env.example documenting the env vars
docker-compose consumes (the real .env stays gitignored).

Test plan

Verified locally:

• ✅ Native linux/amd64 (Linux x86_64): builds clean. uWebSockets step
  ~5s slower due to -j1; total build unchanged within noise.
• ✅ Apple Silicon under DOCKER_DEFAULT_PLATFORM=linux/amd64: builds
  to completion (previously died at step 12/76).

export DOCKER_DEFAULT_PLATFORM=linux/amd64
docker compose build search-engine
# ...
# => exporting to image
# => => exporting layers
# Successfully built search-engine

Risk

Low. The pinned tags are public stable releases (v20.74.0, v0.8.6); the
-j1 change applies to a single ~30-second compile step. Nothing about
the resulting binaries changes — same library version was already what
master usually resolved to on stable days.

If a contributor wants newer uWS, they pass --build-arg UWEBSOCKETS_TAG=....