feat(docker): allow setting user and group id via env

Dockerfile

@@ -8,7 +8,9 @@ LABEL org.opencontainers.image.authors="Gareth Flowers" \
 	org.opencontainers.image.vendor="garethflowers"
 
 ENV FTP_USER=foo \
-	FTP_PASS=bar
+	FTP_PASS=bar \
+	GID=1000 \
+	UID=1000
 
 RUN apk add --no-cache --update \
 	vsftpd==3.0.3-r6

src/docker-entrypoint.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 
 addgroup \
-	-g 1000 \
+	-g $GID \
 	-S \
 	$FTP_USER
 
@@ -10,7 +10,7 @@ adduser \
 	-G $FTP_USER \
 	-h /home/$FTP_USER \
 	-s /bin/false \
-	-u 1000 \
+	-u $UID \
 	$FTP_USER
 
 mkdir -p /home/$FTP_USER