docker: add send payment alice->bob workflow for newcomers

docker/btcd/start-btcctl.sh

@@ -0,0 +1,37 @@
+#!/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() {
+    # docker initialized env variables with blank string and we can't just
+    # use -z flag as usually.
+    BLANK_STRING='""'
+
+    VARIABLE="$1"
+    NAME="$2"
+    DEFAULT="$3"
+
+    if [[ -z "$VARIABLE" || "$VARIABLE" == "$BLANK_STRING" ]]; then
+
+        if [ -z "$DEFAULT" ]; then
+            echo "You should specify '$NAME' env variable"
+            exit 0
+        else
+            VARIABLE="$DEFAULT"
+        fi
+    fi
+
+    # echo is used as return in case if string values
+    echo "$VARIABLE"
+}
+
+RPCUSER=$(set_env "$RPCUSER" "RPCUSER")
+RPCPASS=$(set_env "$RPCPASS" "RPCPASS")
+
+btcctl \
+    --simnet \
+    --rpccert="/rpc/rpc.cert" \
+    --rpcuser="$RPCUSER" \
+    --rpcpass="$RPCPASS" \
+    --rpcserver="rpcserver" \
+    "$@"