tests/docker: inline variable definitions or move close to use

tests/docker: inline variable definitions or move close to use

Variables that are defined with ":=" are handled imperatively, so moving
them closer to the first use ensures that all the assignments prior to
the first use are taken into account.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220401141326.1244422-6-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220419091020.3008144-9-alex.bennee@linaro.org>