Replace Bash $UID with portable $(id -u) in Docker scripts

@Pentarctagon noticed in commit 9a34cbb329 that $UID isn't set when
/bin/sh isn't GNU Bash.  `id -u` is specified by POSIX:
https://pubs.opengroup.org/onlinepubs/9699919799.2018edition/utilities/id.html

Also move `-xe` to a `set` command (in case someone runs scripts with
`sh path/to/script` instead of `path/to/script`) and add `set -u` to
complain more loudly about such unset variables.

utils/dockerbuilds/make_mingw_build

@@ -1,7 +1,9 @@
-#!/bin/sh -xe
+#!/bin/sh
+
+set -xeu
 
 cd mingw
 docker build -t mingw-wesnoth .
 rm -rf ../mingwbuild
 mkdir ../mingwbuild
-docker run -v "$PWD"/../../..:/wesnoth -v "$PWD"/../mingwbuild:/output --tmpfs /build -u "$UID" mingw-wesnoth
+docker run -v "$PWD"/../../..:/wesnoth -v "$PWD"/../mingwbuild:/output --tmpfs /build -u "$(id -u)" mingw-wesnoth

utils/dockerbuilds/make_steam_build

@@ -1,4 +1,6 @@
-#!/bin/sh -xe
+#!/bin/sh
+
+set -xeu
 
 cd steamrt
 docker build -t steamos-wesnoth .
@@ -7,4 +9,4 @@ mkdir ../steambuild
 if [ -d "$PWD"/../../../build ]; then
     echo "Existing scons 'build' directory found, this may cause the build to fail!"
 fi
-docker run -v "$PWD"/../../..:/wesnoth -v "$PWD"/../steambuild:/output --tmpfs /build:exec -u "$UID" steamos-wesnoth
+docker run -v "$PWD"/../../..:/wesnoth -v "$PWD"/../steambuild:/output --tmpfs /build:exec -u "$(id -u)" steamos-wesnoth

utils/dockerbuilds/make_steam_image

@@ -1,4 +1,6 @@
-#!/bin/sh -xe
+#!/bin/sh
+
+set -xeu
 
 VALVEFILE=com.valvesoftware.SteamRuntime.Sdk-amd64,i386-sniper-sysroot.Dockerfile
 VALVETAR=com.valvesoftware.SteamRuntime.Sdk-amd64,i386-sniper-sysroot.tar.gz

utils/itchio_upload

@@ -1,4 +1,6 @@
-#!/bin/sh -xe
+#!/bin/sh
+
+set -xeu
 
 # Run this script from directory containing windows installer(s)
 # passing needed version as argument