Install envsubst in Dockerfile

@ulfgebhardt please setup docker on your machine or a remote machine.

Installing `envsubst` on alpine fails with circular dependencies
(awkward). So this repo here has a solution:
https://github.com/cirocosta/alpine-envsubst/blob/master/Dockerfile#L6

deployment/legacy-migration/maintenance-worker/Dockerfile

@@ -3,6 +3,16 @@ FROM humanconnection/neo4j:latest
 ENV NODE_ENV=maintenance
 EXPOSE 7687 7474
 
+ENV BUILD_DEPS="gettext"  \
+    RUNTIME_DEPS="libintl"
+
+RUN set -x && \
+    apk add --update $RUNTIME_DEPS && \
+    apk add --virtual build_deps $BUILD_DEPS &&  \
+    cp /usr/bin/envsubst /usr/local/bin/envsubst && \
+    apk del build_deps
+
+
 RUN apk upgrade --update
 RUN apk add --no-cache mongodb-tools openssh nodejs yarn rsync