Migrate pre-commit.entrypoint.sh to POSIX sh (#5540)

The script was basically posix with a bashism.
POSIX sh is lighter and thus better.

Reviewed-on: #5540
Reviewed-by: delliott <delliott@fsfe.org>

pre-commit.Dockerfile

@@ -45,4 +45,4 @@ COPY pre-commit.entrypoint.sh ./
 # Set the workdir
 WORKDIR /website-source
 
-ENTRYPOINT ["bash", "/website-source-during-build/pre-commit.entrypoint.sh"]
+ENTRYPOINT ["sh", "/website-source-during-build/pre-commit.entrypoint.sh"]

pre-commit.entrypoint.sh

@@ -1,5 +1,5 @@
-#!/usr/bin/env bash
-set -euo pipefail
+#!/bin/sh
+set -eu
 
 # Get the changed files
 target_branch="${1:-master}"
@@ -8,7 +8,7 @@ files="$(git diff --name-only "$source_branch" "$target_branch")"
 files_args=""
 for file in $files; do
 	if [ -f "$file" ]; then
-		files_args+="--file $file "
+		files_args="$files_args --file $file"
 	fi
 done