Bug 1846: 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 1846

Change-Id: I589199979818b0fe5892df1d40849da8f6e06ded
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/requirements-dev.txt b/requirements-dev.txt
index 0151049..6d6c3af 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -1,34 +1,3 @@
-bitarray==1.8.1
-    # via
-    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
-    #   pyangbind
-enum34==1.1.10
-    # via
-    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
-    #   pyangbind
-lxml==4.6.3
-    # via
-    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
-    #   pyang
-    #   pyangbind
-git+https://osm.etsi.org/gerrit/osm/IM.git@master#egg=osm-im
-    # via -r requirements-dev.in
-pyang==2.4.0
-    # via
-    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
-    #   pyangbind
-pyangbind==0.8.1
-    # via -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
-pyyaml==5.4.1
-    # via -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
-regex==2021.3.17
-    # via
-    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
-    #   pyangbind
-six==1.15.0
-    # via
-    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
-    #   pyangbind
 #######################################################################################
 # Copyright ETSI Contributors and Others.
 #
@@ -45,3 +14,34 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #######################################################################################
+bitarray==2.3.5
+    # via
+    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
+    #   pyangbind
+enum34==1.1.10
+    # via
+    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
+    #   pyangbind
+lxml==4.7.1
+    # via
+    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
+    #   pyang
+    #   pyangbind
+osm-im @ git+https://osm.etsi.org/gerrit/osm/IM.git@master
+    # via -r requirements-dev.in
+pyang==2.5.2
+    # via
+    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
+    #   pyangbind
+pyangbind==0.8.1
+    # via -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
+pyyaml==5.4.1
+    # via -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
+regex==2021.11.10
+    # via
+    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
+    #   pyangbind
+six==1.16.0
+    # via
+    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=master
+    #   pyangbind