Check dev container (container) properties before attempting to modify CAP_NET_ADMIN

[mafredri]

Right now we try to set CAP_NET_ADMIN on the agent binary within the dev container (similar to our bootstrap scripts). This is to improve network performance, but we could probably skip the attempt if we know the container won't support it and thus not print a warning.

See agent/agentcontainers/api.go.

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This will probably fail unless the container is running as privileged or has the specific CAP_NET_ADMIN privilege set on the container?

Originally posted by @johnstcn in coder/coder#18245 (comment)