Dockerfile: use env vars for consistency

Signed-off-by: Lakshya Singh <lakshay.singh1108@gmail.com>

lightningd/Dockerfile

@@ -146,9 +146,9 @@ COPY --from=builder /usr/local/bin/summars /usr/local/src/plugins/summars
 COPY --from=vls_builder /usr/local/bin/remote_hsmd_socket /usr/libexec/c-lightning/remote_hsmd_socket
 COPY --from=builder /build/core-lightning/plugins/clnrest/requirements.txt /usr/local/src/plugins/clnrest_requirements.txt
 
-RUN addgroup -S lightning && adduser -S lightning -G lightning && \
+RUN addgroup -S ${LIGHTNINGD_USER} && adduser -S ${LIGHTNINGD_USER} -G ${LIGHTNINGD_USER} && \
     mkdir -p ${LIGHTNINGD_DATA} && \
-    chown -R lightning:lightning ${LIGHTNINGD_DATA}
+    chown -R ${LIGHTNINGD_USER}:${LIGHTNINGD_USER} ${LIGHTNINGD_DATA}
 
 COPY bitcoin.conf ${BITCOIND_HOME}/bitcoin.conf
 
@@ -161,13 +161,13 @@ RUN chmod +x /healthcheck.sh
 VOLUME ["${LIGHTNINGD_DATA}"]
 
 RUN mkdir -p ${BITCOIND_HOME} && \
-    chown -R lightning:lightning ${BITCOIND_HOME}
+    chown -R ${LIGHTNINGD_USER}:${LIGHTNINGD_USER} ${BITCOIND_HOME}
 
 COPY bitcoin.conf ${BITCOIND_HOME}/bitcoin.conf
 COPY testnet-config /testnet-config
 COPY regtest-config /regtest-config
 
-USER lightning
+USER ${LIGHTNINGD_USER}
 
 RUN pip3 install \
       pyln-client \