Unify entrypoint scripts for 5.7 and 8.0

There were only two differences between the scripts:
* 8.0 had a fix for datadir settings with spaces in them
* 8.0 used alter user to set the root password, which 5.7 should also use

5.7/docker-entrypoint.sh

@@ -77,7 +77,9 @@ _check_config() {
 # latter only show values present in config files, and not server defaults
 _get_config() {
 	local conf="$1"; shift
-	"$@" --verbose --help --log-bin-index="$(mktemp -u)" 2>/dev/null | awk '$1 == "'"$conf"'" { print $2; exit }'
+	"$@" --verbose --help --log-bin-index="$(mktemp -u)" 2>/dev/null \
+		| awk '$1 == "'"$conf"'" && /^[^ \t]/ { sub(/^[^ \t]+[ \t]+/, ""); print; exit }'
+	# match "datadir      /some/path with/spaces in/it here" but not "--xyz=abc\n     datadir (xyz)"
 }
 
 # allow the container to be started with `--user`
@@ -161,7 +163,7 @@ if [ "$1" = 'mysqld' -a -z "$wantHelp" ]; then
 			--  or products like mysql-fabric won't work
 			SET @@SESSION.SQL_LOG_BIN=0;
 
-			SET PASSWORD FOR 'root'@'localhost'=PASSWORD('${MYSQL_ROOT_PASSWORD}') ;
+			ALTER USER 'root'@'localhost' IDENTIFIED BY '${MYSQL_ROOT_PASSWORD}' ;
 			GRANT ALL ON *.* TO 'root'@'localhost' WITH GRANT OPTION ;
 			${rootCreate}
 			DROP DATABASE IF EXISTS test ;