Bug 1844: LTS Support

Updates base to Ubuntu 20.04
Updates python to 3.8
Updates pip dependencies to more recent versions
Adds apt cache to stage 2 dockerfile

Fixes bug 1844

Change-Id: Iccea9a5121fa8e701ae18c64d71d094ee7c535c7
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/requirements.in b/requirements.in
index 24f4c58..94c2eb9 100644
--- a/requirements.in
+++ b/requirements.in
@@ -15,4 +15,4 @@
 
 pyang
 pyangbind
-pyyaml
\ No newline at end of file
+pyyaml==5.4.1
\ No newline at end of file