Add UID as env in Dockerfile [skip ci]

pica-oh-my-form/Dockerfile

@@ -2,8 +2,10 @@ FROM  node:12.9.1-alpine
 LABEL maintainer="quentinduchemin@tuta.io"
 
 ARG OHMYFORM_VERSION=0.2.3
+# Probably does not need to change, bound to LDAP UID
+ARG FORM_UID=5002
 # Create a group and a user with name "ohmyform".
-RUN addgroup --gid 9999 ohmyform && adduser -D --uid 9999 -G ohmyform ohmyform
+RUN addgroup --gid ${FORM_UID} ohmyform && adduser -D --uid ${FORM_UID} -G ohmyform ohmyform
 
 # Install some needed packages
 RUN apk add --no-cache git python=2.7.16-r1 \