Bug 1850: 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 1850

Change-Id: I29dcb5862b26021cecb4e581bd3cbd870682c968
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/requirements.in b/requirements.in
index e72b2cc..9bac01f 100644
--- a/requirements.in
+++ b/requirements.in
@@ -13,12 +13,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 ##
-bitarray==1.8.1
-charm-tools
+bitarray
 haikunator
 pyangbind
 python-openstackclient
-regex==2021.3.17
+pyyaml<6
+regex
 requests
 robotframework
 robotframework-jsonlibrary