docker: general improvements

* rename set_env->set_default
* fix bug with return from bash func
* make '--miningaddr' optional

docker/btcd/start-btcctl.sh

@@ -1,32 +1,46 @@
 #!/usr/bin/env bash
 
-# Check env variable and in case of empty value and default value specified
-# returns default value, in case of non-empty value returns value.
-set_env() {
+# exit from script if error was raised.
+set -e
+
+# error function is used within a bash function in order to send the error
+# message directly to the stderr output and exit.
+error() {
+    echo "$1" > /dev/stderr
+    exit 0
+}
+
+# return is used within bash function in order to return the value.
+return() {
+    echo "$1"
+}
+
+# set_default function gives the ability to move the setting of default
+# env variable from docker file to the script thereby giving the ability to the
+# user override it durin container start.
+set_default() {
     # docker initialized env variables with blank string and we can't just
     # use -z flag as usually.
     BLANK_STRING='""'
 
     VARIABLE="$1"
-    NAME="$2"
-    DEFAULT="$3"
+    DEFAULT="$2"
 
     if [[ -z "$VARIABLE" || "$VARIABLE" == "$BLANK_STRING" ]]; then
 
         if [ -z "$DEFAULT" ]; then
-            echo "You should specify '$NAME' env variable"
-            exit 0
+            error "You should specify default variable"
         else
             VARIABLE="$DEFAULT"
         fi
     fi
 
-    # echo is used as return in case if string values
-    echo "$VARIABLE"
+   return "$VARIABLE"
 }
 
-RPCUSER=$(set_env "$RPCUSER" "RPCUSER")
-RPCPASS=$(set_env "$RPCPASS" "RPCPASS")
+# Set default variables if needed.
+RPCUSER=$(set_default "$RPCUSER" "devuser")
+RPCPASS=$(set_default "$RPCPASS" "devpass")
 
 btcctl \
     --simnet \