add ability to substitute env vars with _file vars

docker/entrypoint.sh

@@ -1,6 +1,41 @@
 #!/usr/bin/env sh
 set -e
 
+# usage: file_env VAR [DEFAULT]
+#   Set $VAR to the contents of the path specified by $VAR_FILE. Useful for docker secrets
+file_env() {
+  VAR="$1"
+  FILE_VAR="${VAR}_FILE"
+  eval "VAR_EXPANDED=\"\${$VAR}\""
+  eval "FILE_VAR_EXPANDED=\"\${$FILE_VAR}\""
+  DEFAULT="${2:-}"
+
+  if [ "${VAR_EXPANDED:-}" ] && [ "${FILE_VAR_EXPANDED:-}" ]; then
+    echo >&2 "error: both $VAR and $FILE_VAR are set (but are exclusive)"
+    exit 1
+  fi
+
+  VAL="$DEFAULT"
+  if [ "${VAR_EXPANDED:-}" ]; then
+    VAL="${VAR_EXPANDED}"
+  elif [ "${FILE_VAR_EXPANDED:-}" ]; then
+    if [ -f "${FILE_VAR_EXPANDED}" ]; then
+      VAL="$(cat "${FILE_VAR_EXPANDED}")"
+    else
+      echo >&2 "error: couldn't find file at '$FILE_VAR_EXPANDED'"
+      exit 1
+    fi
+  fi
+
+  export "$VAR"="$VAL"
+  unset "$FILE_VAR"
+}
+
+# Substitue all env vars starting with VIKUNJA and ending with _FILE
+for var in $(env | sed -n "s/^\(VIKUNJA.*\)_FILE=.*$/\1/p"); do
+  file_env "${var}"
+done
+
 if [ -n "$PUID" ] && [ "$PUID" -ne 0 ] && \
    [ -n "$PGID" ] && [ "$PGID" -ne 0 ] ; then
   echo "info: creating the new user vikunja with $PUID:$PGID"