travis: set the `container` env variable explicitly

As we build a custom Docker image, we need to ensure that systemd
correctly detects it's running in a container. One can simply do this
directly in a Dockerfile (ENV container docker) or when starting the
container (-e container=docker). For the sake of readability, let's
pick the latter approach here.

travis-ci/managers/debian.sh

@@ -32,7 +32,7 @@ for phase in "${PHASES[@]}"; do
             info "Using Debian $DEBIAN_RELEASE"
             printf "FROM debian:$DEBIAN_RELEASE\nRUN bash -c 'apt-get -y update && apt-get install -y systemd'\n" | docker build -t debian-with-systemd/latest -
             info "Starting container $CONT_NAME"
-            $DOCKER_RUN -v $REPO_ROOT:/build:rw \
+            $DOCKER_RUN -v $REPO_ROOT:/build:rw -e container=docker \
                         -w /build --privileged=true --name $CONT_NAME \
                         -dit --net=host debian-with-systemd/latest /bin/systemd
             $DOCKER_EXEC bash -c "echo deb-src http://deb.debian.org/debian $DEBIAN_RELEASE main >>/etc/apt/sources.list"