github: Fix warnings about Dockerfile syntax

.github/workflows/Dockerfile.ci.alpine

@@ -1,6 +1,6 @@
 FROM localhost:5000/lldap/lldap:alpine-base
 # Taken directly from https://github.com/tianon/gosu/blob/master/INSTALL.md
-ENV GOSU_VERSION 1.17
+ENV GOSU_VERSION=1.17
 RUN set -eux; \
 	\
 	apk add --no-cache --virtual .gosu-deps \

.github/workflows/Dockerfile.ci.debian

@@ -1,6 +1,6 @@
 FROM localhost:5000/lldap/lldap:debian-base
 # Taken directly from https://github.com/tianon/gosu/blob/master/INSTALL.md
-ENV GOSU_VERSION 1.17
+ENV GOSU_VERSION=1.17
 RUN set -eux; \
 # save list of currently installed packages for later so we can clean up
 	savedAptMark="$(apt-mark showmanual)"; \