Dockefile: use base image 'ubuntu:latest' instead of focal

Default python version in focal is python3.8 which is breaking
integration CI builds on t-api PRs with:
`ERROR: Package 'teuthology-1.1.1.dev729-g861a8dcf' requires a different Python: 3.8.10 not in '>=3.10'`

Recently, we upgraded the minimum teuthology version to support
python3.10, which could be the reason of the above CI failures.

Signed-off-by: Vallari Agrawal <[email protected]>

Dockerfile

@@ -1,5 +1,6 @@
-FROM ubuntu:focal
+FROM ubuntu:latest
 ENV DEBIAN_FRONTEND=noninteractive
+ENV VENV=${VENV:-"venv"}
 RUN apt-get update && \
     apt-get install -y \
     git \
@@ -21,7 +22,8 @@ RUN apt-get update && \
 COPY .teuthology.yaml /root
 WORKDIR /teuthology_api
 COPY . /teuthology_api/
-RUN pip3 install -e .
+RUN python3 -m venv .${VENV}
+RUN .${VENV}/bin/pip3 install -e .
 RUN mkdir /archive_dir/
 
 CMD sh /teuthology_api/start_container.sh