v0.7.1: add some env/container specs

What's Changed

• feat(env/container): add kernel.pid_max field to KernelSysctl struct by @ssst0n3 in #124
• refactor(env/container): restructure kernel handling with new subpack… by @ssst0n3 in #125
• refactor(rlimit): simplify Rlimit struct by removing Resource wrapper by @ssst0n3 in #126
• test(env/container): add threads_max field by @ssst0n3 in #127
• feat(env/container): add PidsMax field to CGroups to represent proces… by @ssst0n3 in #128
• refactor(prerequisite): simplify CheckTemplate to use in-place field … by @ssst0n3 in #129

Full Changelog: v0.7.0...v0.7.1