Update Dockerfile.ubuntu22.04

[observerw]

Contains two necessary changes to build successfully:

1. Move DEBIAN_FRONTEND to avoid timezone interaction. Current ARG DEBIAN_FRONTEND not seems to work.
2. Install libfl-dev, otherwise OpenSTA make will failed:

0.717 CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
0.717 Please set them or make sure they are set and tested correctly in the CMake files:
0.717 /OpenSTA/FLEX_INCLUDE_DIR
0.717    used as include directory in directory /OpenSTA

[CLAassistant]

All committers have signed the CLA.

[maliberty]

@dsengupta0628 not sure if this is still relevant or not

[dsengupta0628]

Checked this against current master - I don't think it's still needed.

Both changes were verified in Docker against the current Dockerfile.ubuntu22.04:

1. ARG DEBIAN_FRONTEND

It does work. A minimal build confirms the build arg is exported into the RUN environment:

[2/2] RUN echo "FRONTEND=[$DEBIAN_FRONTEND]"
FRONTEND=[noninteractive]

Building the current apt stage as-is completes with no tzdata prompt.

2. libfl-dev / FLEX_INCLUDE_DIR NOTFOUND

libfl-dev is a Recommends of the flex package on Ubuntu 22.04:

$ apt-cache show flex | grep Recommends
Recommends: gcc | c-compiler, libfl-dev

So the current apt-get install -y flex already pulls it in, and /usr/include/FlexLexer.h is present - confirmed by building the existing apt stage. The NOTFOUND error only appears once --no-install-recommends is added, which is the other half of this PR. In other words, the missing libfl-dev is caused by the change itself rather than being a pre-existing bug.