bump relx to v4.2.0 and rebar3 version to 3.14.3

Add support for relx config file

create release and upload escript for new tags

output relative paths in dialyzer provider

bump relx to v4.1.0

Prevent dialyzer warnings on OTP 23

We adapt .gitignore to the new vendored folder, also

1. we now rely on $1's vendor.sh script (instead of duplicating it here)
2. we assume that hex_core is rebar3-compatible
3. we rely on TARGET_ERLANG_VERSION
4. we generate exactly what was previously generating, while targeting a specific Erlang/OTP vsn

(some are not "wrong" per se, but incomplete)

Add a docker recipe for rebar3 itself

fix logger reset for the shell plugin

Re-vendor hex_core in at 0.7.0

The shell plugin has to reset the logger by fetching its configuration and
re-applying it later. In the current state, `maybe_reset_logger/1` will only
re-apply part of the configuration (the `config` key), which does not include
other settings such as filters and formatters.

The effect can be demonstrated by adding filters or formatters to `~/.erlang`
and running rebar3 shell on any project (without any --config argument, this
one reload the logger env; updating the kernel/logger environment in
`~/.erlang` fixes it).

For example:
```erlang
logger:set_handler_config(default, #{formatter => {logger_formatter,
                                                   #{template => ["> ", msg, "\n"]}}}).
```

This patch makes sure the default handler is re-created with its entire
original configuration.

Fix relup cmd arguments and add validation

relx:build_relup does not allow for undefined Release or ToVsn
so we verify that they have been given on the command line
before running relx:build_relup/4.

Based on the closest Alpine Erlang version currently supported by
rebar3. Allows people to mix and match different versions of
the Erlang VM and rebar3 using multi-stage builds.

Attempt to add checkout location to rebar3 path

Fix spec for download callback

Add rebar_hex_repos:remove_from_auth_config/2

- add function that allows the complete removal of an entry from auth
 config

 - add test for rebar_hex_repos:remove_from_auth_config/2

 - update test/rebar_pkg_repos_SUITE:auth_read_write_read/1 to use mocks so
 we don't append to actual hex.config files

- update bootstrap script to account for hex_core in src/vendored