The `docker` CLI currently doesn't handle situations where the current context
(as defined in `~/.docker/config.json`) is invalid or doesn't exist. As loading
(and checking) the context happens during initialization of the CLI, this
prevents `docker context` commands from being used, which makes it complicated
to fix the situation. For example, running `docker context use <correct context>`
would fail, which makes it not possible to update the `~/.docker/config.json`,
unless doing so manually.

For example, given the following `~/.docker/config.json`:

```json
{
        "currentContext": "nosuchcontext"
}
```

All of the commands below fail:

```bash
docker context inspect rootless
Current context "nosuchcontext" is not found on the file system, please check your config file at /Users/thajeztah/.docker/config.json

docker context rm --force rootless
Current context "nosuchcontext" is not found on the file system, please check your config file at /Users/thajeztah/.docker/config.json

docker context use default
Current context "nosuchcontext" is not found on the file system, please check your config file at /Users/thajeztah/.docker/config.json
```

While these things should be fixed, this patch updates the script to switch
the context using the `--context` flag; this flag is taken into account when
initializing the CLI, so that having an invalid context configured won't
block `docker context` commands from being executed. Given that all `context`
commands are local operations, "any" context can be used (it doesn't need to
make a connection with the daemon).

With this patch, those commands can now be run (and won't fail for the wrong
reason);

```bash
 docker --context=default context inspect -f "{{.Name}}" rootless
rootless

docker --context=default context inspect -f "{{.Name}}" rootless-doesnt-exist
context "rootless-doesnt-exist" does not exist
```

One other issue may also cause things to fail during uninstall; trying to remove
a context that doesn't exist will fail (even with the `-f` / `--force` option
set);

```bash
docker --context=default context rm blablabla
Error: context "blablabla": not found
```

While this is "ok" in most circumstances, it also means that (potentially) the
current context is not reset to "default", so this patch adds an explicit
`docker context use`, as well as unsetting the `DOCKER_HOST` and `DOCKER_CONTEXT`
environment variables.

Signed-off-by: Sebastiaan van Stijn <[email protected]>