Static linking for docker tests

[noname0443]

When we compile mysync on local machine we have a chance that our own glibc will be newer then on ubuntu's 20.04. In this case we can't correctly run mysync and will get error:

/usr/bin/mysync: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not found (required by /usr/bin/mysync)
/usr/bin/mysync: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by /usr/bin/mysync)
/usr/bin/mysync: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not found (required by /usr/bin/mysync)
/usr/bin/mysync: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by /usr/bin/mysync)
/usr/bin/mysync: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not found (required by /usr/bin/mysync)
/usr/bin/mysync: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by /usr/bin/mysync)
/usr/bin/mysync: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not found (required by /usr/bin/mysync)
/usr/bin/mysync: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by /usr/bin/mysync)

I suggest to link glibc for mysync statically for docker tests to evade this problem.