build: add build.sh convenience script for linux

This makes it easier to do rebuilds using docker, because it doesn't
actually modify the source tree (so that Git actually understands what
you are doing with the chmodding), as all directories touched by docker
are temporary, except for `result`, which is the "output". This just
contains ISLE.EXE, LEGO1.DLL and LEGO1.lib.

Optionally Nix can be used to speed up the "copy the source to somewhere
temporary" step.

Note that this doesn't work for MacOS, because this script uses
`--tmpfs`, which is Linux-only.

docker/build.sh

@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+
+set -euxo pipefail
+
+# two `cd`'s so Git can find the root dir even if this script is called from
+# outside the repo
+cd $(dirname "${BASH_SOURCE[0]}")
+cd $(git rev-parse --show-toplevel)
+
+# nix-store is way quicker at copying these files and only needs to do it once
+# if the source doesn't change
+if whereis nix-store >/dev/null; then
+    SRCDIR="$(nix-store --add $PWD)"
+else
+    SRCDIR="$(mktemp -d /tmp/isle-src.XXXXXX)"
+    git ls-files -z | xargs -0I{} install -D {} "$SRCDIR/{}"
+fi
+
+cleanup () {
+    rm -rf "$SRCDIR" 2>/dev/null
+}
+
+trap cleanup EXIT
+
+declare oci_cmd
+
+if whereis podman >/dev/null; then
+    oci_cmd=podman
+elif whereis docker >/dev/null; then
+    oci_cmd=docker
+else
+    echo "No container engine (docker/podman) found!"
+    exit 2
+fi
+
+"$oci_cmd" build -t isle "$SRCDIR/docker"
+
+mkdir -p result
+rm -rf result/*
+
+if [[ "x${JOBS:-}" == "x" ]]; then
+    JOBS=$(nproc)
+fi
+
+"$oci_cmd" run -it \
+    -e CMAKE_FLAGS="-DCMAKE_BUILD_TYPE=RelWithDebInfo" \
+    -v "$SRCDIR":/isle:rw \
+    -e JOBS="$JOBS" \
+    --tmpfs /build \
+    -v ./result:/install \
+    isle
+