Install requirements via install script

Closes https://git.autonomic.zone/coop-cloud/abra/issues/196.

.drone.yml

@@ -22,6 +22,7 @@ steps:
   - name: test installation script
     image: debian:buster
     commands:
+      - apt update && apt install -yqq sudo
       - deploy/install.abra.coopcloud.tech/installer --no-prompt
 
   - name: publish image

deploy/install.abra.coopcloud.tech/installer

@@ -194,6 +194,8 @@ function install_abra_dev {
 }
 
 function run_installation {
+  show_banner
+
   DOCOPT_PREFIX=installer_
   DOCOPT_ADD_HELP=false
   eval "$(docopt "$@")"
@@ -202,17 +204,19 @@ function run_installation {
   no_prompt="$installer___no_prompt"
   no_deps="$installer___no_deps"
 
-  show_banner
+  if [ "$no_deps" == "false" ]; then
+    install_requirements
+  fi
+
+  if ! type curl > /dev/null 2>&1; then
+    error "'curl' program is not installed, cannot continue..."
+  fi
 
   if [ "$dev" == "true" ]; then
     install_abra_dev
   else
     install_abra_release
   fi
-
-  if [ "$no_deps" == "false" ]; then
-    install_requirements
-  fi
 }
 
 run_installation "$@"