contrib: Dockerfile: install libnss-unknown

While Podman automatically populates /etc/passwd and /etc/group with
matching entries for the mapped host user, such a feature is missing
from Docker. Install libnss-unknown, which will return passwd entries of
the form "uid-1000" for all unknown UIDs, to avoid warnings and other
weird behaviour that may occur when running as an unnamed user.

`-o` must be passed to useradd now to allow non-unique user entries, as
with libnss-unknown, every UID is considered occupied.

contrib/docker/Dockerfile

@@ -21,6 +21,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     ecdsautils \
     lua-check \
     shellcheck \
+    libnss-unknown \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*
 
@@ -30,7 +31,7 @@ RUN mkdir /tmp/ec &&\
     mv bin/ec-linux-amd64 /usr/local/bin/editorconfig-checker &&\
     rm -rf /tmp/ec
 
-RUN useradd -m -d /gluon -u 100 -g 100 gluon
+RUN useradd -m -d /gluon -u 100 -g 100 -o gluon
 USER gluon
 
 VOLUME /gluon