set an environment variable to indicate running in docker

the Recorder code will check this and if it's set use
  openlog(LOG_PERROR) for stderr messages

  addresses https://github.com/owntracks/recorder/issues/426

Dockerfile

@@ -3,6 +3,8 @@ FROM alpine:3.16 AS builder
 ARG RECORDER_VERSION=0.9.3
 # ARG RECORDER_VERSION=master
 
+ENV DOCKER_RUNING=1
+
 RUN apk add --no-cache \
         make \
         gcc \
@@ -26,6 +28,8 @@ RUN make install DESTDIR=/app
 
 FROM alpine:3.16
 
+ENV DOCKER_RUNING=1
+
 VOLUME ["/store", "/config"]
 
 RUN apk add --no-cache \