ci: Use Dockerfile to define build environment

The basic Jenkins agent does not have the *python3-wheel* package
installed, which is required to fetch the dependencies for this web
application. Thus, a specific environment has to be defined that
includes all of the required tools.

ci/Dockerfile

@@ -0,0 +1,7 @@
+FROM fedora
+
+RUN dnf install -y \
+	openssh-clients \
+	python3-pip \
+	python3-wheel \
+	rsync

ci/Jenkinsfile

@@ -1,5 +1,9 @@
 pipeline {
-    agent any
+    agent {
+        dockerfile {
+            dir 'ci'
+        }
+    }
 
     triggers {
         pollSCM ''