Fix "git-unknown" in --version of unstable docker image

.travis.yml

@@ -86,7 +86,8 @@ matrix:
             - if [[ "$ATTEMPT_DEPLOY" == "yes" ]]; then echo "$DOCKER_PASSWORD" | docker login -u "$DOCKER_USERNAME" --password-stdin; fi
             - export SECRET_KEY=no DOCKER_PASSWORD=no DOCKER_USERNAME=no
           script:
-            - if [[ "$ATTEMPT_DEPLOY" == "yes" ]]; then docker build --build-arg BUILD_DATE=`date -u +"%Y-%m-%dT%H:%M:%SZ"` --build-arg VCS_REF=`git rev-parse HEAD` -t "zncbouncer/znc-git:$TRAVIS_BRANCH" -t "zncbouncer/znc-git:$TRAVIS_BRANCH-$(git describe)" .; fi
+            - echo "$TRAVIS_BRANCH-$(git-describe)" > .nightly
+            - if [[ "$ATTEMPT_DEPLOY" == "yes" ]]; then docker build --build-arg BUILD_DATE=`date -u +"%Y-%m-%dT%H:%M:%SZ"` --build-arg VCS_REF=`git rev-parse HEAD` --build-arg VERSION_EXTRA=+docker-git- -t "zncbouncer/znc-git:$TRAVIS_BRANCH" -t "zncbouncer/znc-git:$TRAVIS_BRANCH-$(git describe)" .; fi
             - if [[ "$ATTEMPT_DEPLOY" == "yes" && "$TRAVIS_BRANCH" == "master" ]]; then docker tag "zncbouncer/znc-git:$TRAVIS_BRANCH" zncbouncer/znc-git:latest; fi
             - if [[ "$ATTEMPT_DEPLOY" == "yes" ]]; then docker push zncbouncer/znc-git; fi
           after_success:

Dockerfile

@@ -1,8 +1,10 @@
 FROM alpine:3.7
 
+ARG VERSION_EXTRA=""
+
 # musl silently doesn't support AI_ADDRCONFIG yet, and ZNC doesn't support Happy Eyeballs yet.
 # Together they cause very slow connection. So for now IPv6 is disabled here.
-ARG CMAKEFLAGS="-DCMAKE_INSTALL_PREFIX=/opt/znc -DWANT_CYRUS=YES -DWANT_PERL=YES -DWANT_PYTHON=YES -DWANT_IPV6=NO"
+ARG CMAKEFLAGS="-DVERSION_EXTRA=${VERSION_EXTRA} -DCMAKE_INSTALL_PREFIX=/opt/znc -DWANT_CYRUS=YES -DWANT_PERL=YES -DWANT_PYTHON=YES -DWANT_IPV6=NO"
 ARG MAKEFLAGS=""
 
 ARG BUILD_DATE