Bug 1847: 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 1847

Change-Id: Icbe9e9efc0edd957a4b8c0c461cf9d32c716df3e
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/requirements.in b/requirements.in
index 82788ed..e76dad1 100755
--- a/requirements.in
+++ b/requirements.in
@@ -15,6 +15,6 @@
 # WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 # License for the specific language governing permissions and limitations
 # under the License.
-pyyaml
+pyyaml==5.4.1
 pymzn==0.18.*
 jinja2