Rework build scripts (close #5)

- Separate cross-compilation logic to crosscompile.vsh
- Manage Docker in make.vsh, no more manual running `docker build`, etc.
- Add MANUAL.md instruction
- Enchance Dockerfile

MANUAL.md

@@ -0,0 +1,138 @@
+# Manual cross-compilation
+
+This repository contains ready-made scripts that allow you to describe the
+desired results and obtain them simply by running `./make.vsh`. However, to
+better understand the process, it's worth considering the manual build.
+This file describes the algorithm of actions automated in the aforementioned
+scripts.
+
+## Prepare the environment
+
+We want reproducible builds. We also don't want to clutter our computer with
+things needed exclusively for cross-compilation. Besides manipulating certain
+packages in the OS can inadvertently damage the system.
+
+Docker will help us achieve all our goals.
+[Install it](https://docs.docker.com/get-started/get-docker/) if you haven't
+already. Containers will ensure reproducible builds, as they will always run
+in the same environment.
+
+## Let's begin
+
+Create a V programm. Just initialize empty V project in some dir:
+
+```console
+$ mkdir crossv
+$ cd crossv
+$ v init
+Input your project description: Cross-compilation example
+Input your project version: (0.0.0)
+Input your project license: (MIT)
+Initialising ...
+Created binary (application) project `crossv`
+```
+
+Contents of `main.v`:
+
+```v
+module main
+
+fn main() {
+        println('Hello World!')
+}
+```
+
+There is already an example Dockerfile in this repository, so here I will focus
+on CLI. So let's run Debian Linux in container with current directory mounted:
+
+```
+docker run --rm -ti -v .:/app -w /app debian:trixie
+```
+
+See https://docs.docker.com/reference/cli/docker/container/run/ for details.
+
+Now we will run shell commands inside container.
+
+## Setup V compiler in container
+
+Install prerequisistes:
+
+```
+apt update
+apt install -y --no-install-recommends --no-install-suggests build-essential git ca-certificates file
+```
+
+Download and bootstrap V compiler:
+
+```
+export VMODULES=/tmp/vmodules VCACHE=/tmp/vcache
+git clone --depth=1 https://github.com/vlang/v /opt/v && make -C /opt/v && /opt/v/v symlink
+```
+
+After this `v` command should work. Try:
+
+```
+v version
+```
+
+## Cross-compile to ARM64 (AArch64)
+
+Your host is most likely an x86_64 computer. For the sake of example, let's
+compile our Linux program for the AArch64 architecture.
+
+First we need to add build requirements. Debian already have an excellent
+[cross-compiling](//wiki.debian.org/CrossCompiling) support.
+
+Prepate Debian package manager:
+
+```
+dpkg --add-architecture arm64
+apt update
+```
+
+We need to install `crossbuild-essential-arm64` package:
+
+```
+apt install -y --no-install-recommends --no-install-suggests crossbuild-essential-arm64
+```
+
+Also there is packages for some other architectures which Debian supports:
+https://packages.debian.org/search?keywords=crossbuild-essential&searchon=names&suite=stable&section=all
+
+Now we have a GCC cross-compiler and some common libraries for ARM64.
+
+To compile out project just run:
+
+```
+v -prod -cc aarch64-linux-gnu-gcc -o hello .
+```
+
+Let's make sure we've built the correct executable using the `file` utility:
+
+```console
+# file hello
+hello: ELF 64-bit LSB pie executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-aarch64.so.1, BuildID[sha1]=e9cfdee9abe5a80c304d489f243fbc60a22d93de, for GNU/Linux 3.7.0, not stripped
+```
+
+Binary is dynamically linked. To produce statically linked binary add `-cflags -static` flag:
+
+```
+v -prod -cc aarch64-linux-gnu-gcc -cflags -static -o hello .
+```
+
+Done.
+
+Since we were operating as the root user inside the container, it's worth
+changing the file owner:
+
+```
+chown 1000:1000 hello
+```
+
+Replace `1000:1000` with your actual `UID:GID` pair on host system.
+
+Now we can exit from container:
+
+```
+exit
+```