feat(agent,pkg): add connector mode to agent

[henrybarreto]

What is the ShellHub Connector?

ShellHub Connector is a new kind of ShellHub Agent that turns
Docker containers into ShellHub Devices. It instances
a new ShellHub Agent, in memory, for each container running, redirecting the SSH
IO, connections in general, and authentication credentials to its file system.

As most of the containers don't have passwords set for its users per default,
it rejects the connection every time for users without credentials.

What is working now?

The initial implementation has support for these kinds of connections:

• Shell
• Exec
• Heredoc

Running Connector

To initialize the ShellHub Connector, enter the agent/ directory, build and
run the agent's binary with connector sub command.

go build -ldflags "-X main.AgentVersion=latest" -o agent-native &&
SERVER_ADDRESS="http://localhost/" PRIVATE_KEYS="/tmp/shellhub/"
TENANT_ID="00000000-0000-4000-0000-000000000000" ./agent-native connector

Environmental variables

To configure the ShellHub Connector, you can/must provide these environmental
variables.

• SERVER_ADDRESS (required) Set the ShellHub server address of the agent will
  use to connect

• PRIVATE_KEYS (required) Specify the path to store the devices/containers
  private keys. If not provided, the agent will generate a new one.

• TENANT_ID (required) Sets the account tenant ID used during communication
  to associate the devices to a specific tenant.

• KEEPALIVE_INTERVAL Determine the interval to send the keep alive message to
  the server. This has a direct impact on the bandwidth used by the device when
  in idle state.

Docker notes

As this implementation uses the Docker Client, you can override the environment
variables provided for Docker in order to change some behaviors, but the
ShellHub Connector doesn't guarantee its right operation with these changes.

Check Docker Client documentation about this for more information.

UI