Ship envsubst with the Docker image to facilitate TOML templating

[neysofu]

envsubst takes care of all foreseeable templating needs that we might face for TOML configuration files.

From all envsubst implementations that I could find, I selected https://github.com/a8m/envsubst because it seemed to be the most complete and well adopted. More fully-fledged templating engines like http://www.confd.io/ are interesting but probably overkill.

Here's an example of a configuration pattern that envsubst can deal with, while our current variable expansion logic can't:

[chains.${CHAIN_0}]
shard = "${SHARD_0}"
provider = [ { label = "${CHAIN_0}", url = "${CHAIN_RPC_0}", features = ["${CHAIN_0_FEATURES}"] } ]

Closes #3946.

[evaporei]

💯