fix: add observer to Dockerfile (#125)

Signed-off-by: Miroslav Bajtoš <oss@bajtos.net>

Dockerfile

@@ -30,6 +30,7 @@ COPY --link package-lock.json package.json ./
 # We cannot use a wildcard until `COPY --parents` is stabilised
 # See https://docs.docker.com/reference/dockerfile/#copy---parents
 COPY --link stats/package.json ./stats/
+COPY --link observer/package.json ./observer/
 
 RUN npm ci --workspaces