[buildbot] docker: fix Dockerfile deprecation warnings

[LEDE Commits]

ynezz pushed a commit to buildbot.git, branch main:
https://git.openwrt.org/c4bcca4c1818dc5235c92d9d5c7b76d12cb3ddef

commit c4bcca4c1818dc5235c92d9d5c7b76d12cb3ddef
Author: Petr Štetiar <ynezz at true.cz>
AuthorDate: Sun Jan 11 07:30:06 2026 +0100

    docker: fix Dockerfile deprecation warnings
    
    Replace deprecated MAINTAINER instruction with LABEL and update ENV
    statements from legacy "ENV key value" format to "ENV key=value" format.
    
    Signed-off-by: Petr Štetiar <ynezz at true.cz>
---
 docker/buildmaster/Dockerfile | 6 +++---
 docker/buildworker/Dockerfile | 8 ++++----
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/docker/buildmaster/Dockerfile b/docker/buildmaster/Dockerfile
index b384245..15899fd 100644
--- a/docker/buildmaster/Dockerfile
+++ b/docker/buildmaster/Dockerfile
@@ -24,14 +24,14 @@ RUN make -j$(nproc) static
 
 
 FROM        debian:11
-MAINTAINER  OpenWrt Maintainers
+LABEL       maintainer="OpenWrt Maintainers"
 
 ARG         DEBIAN_FRONTEND=noninteractive
 ARG         BUILDBOT_VERSION=2.10.1
 ARG         OPENWRT_VERSION=unknown
 
-ENV         BUILDMASTER_CONFIG config.ini
-ENV         BUILDMASTER_PHASE  1
+ENV         BUILDMASTER_CONFIG=config.ini
+ENV         BUILDMASTER_PHASE=1
 
 USER root
 
diff --git a/docker/buildworker/Dockerfile b/docker/buildworker/Dockerfile
index eef2d44..1637a6b 100644
--- a/docker/buildworker/Dockerfile
+++ b/docker/buildworker/Dockerfile
@@ -1,13 +1,13 @@
 FROM        debian:11
-MAINTAINER  OpenWrt Maintainers
+LABEL       maintainer="OpenWrt Maintainers"
 
 ARG         DEBIAN_FRONTEND=noninteractive
 ARG         BUILDBOT_VERSION=2.10.1
 ARG         OPENWRT_VERSION=unknown
 
-ENV         BUILDWORKER_MASTER builds.openwrt.org:9990
-ENV         BUILDWORKER_ADMIN contact at openwrt.org
-ENV         BUILDWORKER_DESCRIPTION Docker Container https://git.openwrt.org/$OPENWRT_VERSION
+ENV         BUILDWORKER_MASTER=builds.openwrt.org:9990
+ENV         BUILDWORKER_ADMIN=contact at openwrt.org
+ENV         BUILDWORKER_DESCRIPTION="Docker Container https://git.openwrt.org/$OPENWRT_VERSION"
 
 USER root