fixup: Dockerfile: reinstate location of config.toml

[jtagcat]

readd WORKDIR /app
config.toml uses relative path, from the current directory, $PWD by default is /.

commit f337f6c
removed WORKDIR /app, and /config.toml was used instead of /app/config.toml.
The change was breaking, unexpected and undocumented.
As users already mount /app/config.toml, revert to using it.

Closes #270

[jtagcat]

Alternatively, PODSYNC_CONFIG_PATH env could be set inside the Dockerfile.

[mxpv]

COPY podsync /podsync needs to be changed as well, so podsync can locate the config file

[jtagcat]

It does not need to be changed, the binary's location does not affect working directory (from what config.toml is taken from).

I tried to test with:

1. docker cp <running image of 2.4.1>:/podsync podsync/

  p.c7.ee:
#    image: mxpv/podsync:v2.4.1
#    build: github.com/jtagcat/podsync#patch-1
    build: podsync/ # git checked out at host $PWD/podsync
    volumes:
      - /barfoo:/app/data:rw
      - /foobar/config.toml:/app/config.toml:ro

it worked (though comments in Dockerfile need to be on newline)

Now, I have no idea how you get the binary in to the Dockerfile, make build builds a binary to bin/podsync, but copying that to the container, running the binary in the container results in standard_init_linux.go:228: exec user process caused: no such file or directory

[jtagcat]

anyw, ready to merge, tested

[mxpv]

Can you do (original version):

WORKDIR /app/
RUN wget ...
COPY podsync /app/podsync

ENTRYPOINT ["/app/podsync"]

data directory path needs to be compatible as well, as I guess it's better to just revert to the original version

[jtagcat]

As requested.

[mxpv]

Thanks