docker: install missing Boost runtime library

[yyasuda]

Fixes #5593

The Docker image currently removes the Boost iostreams runtime
library during image cleanup:

apt-get purge -y ${P4C_DEPS}
apt-get autoremove --purge -y

However, several installed backend executables still depend on:

libboost_iostreams.so.1.71.0

Examples include:

• p4c-bm2-ss
• p4c-ebpf
• p4test
• p4testgen

Reproduction:

docker run --rm -it p4lang/p4c:latest p4c-bm2-ss --help

Result:

error while loading shared libraries:
libboost_iostreams.so.1.71.0: cannot open shared object file

This change explicitly installs the runtime package before cleanup,
so the installed binaries remain executable.

[jafingerhut]

Thanks for doing this. I have seen evidence of this missing library in at least the failing CI test of this PR, which mentions the error message copied below: p4lang/pna#131

p4test: error while loading shared libraries: libboost_iostreams.so.1.71.0: cannot open shared object file: No such file or directory

[jafingerhut]

LGTM

[fruffy]

We should test our docker image to avoid errors such as this one in the future. Also add a note that we remove these installations once the root issue is fixed.