Improve Dockerfile

[janhn]

1. Reduces docker image size from ~860MB to ~470MB.

Running a bare chown (or any other form of file modification, e.g. chmod) recursively on a directory tree in a Dockerfile comes with a cost: docker makes a copy of the whole layer to apply the change. Solvable with --chown argument to COPY.

Changes from:

$ docker image ls | grep electrs                                                             
electrs-app [...] 865MB                                                          

$ docker history electrs-app                                                                 
IMAGE          CREATED         CREATED BY                                      SIZE      COMM
...
fae691841c1a   3 minutes ago   /bin/sh -c groupadd -r user     && adduser -…   393MB
1c4b0ea96c3c   3 minutes ago   /bin/sh -c #(nop) COPY dir:7956596a8fde73ca1…   393MB 

to:

$ docker image ls | grep electrs                                                 
electrs-app [...] 472MB                                                          

$ docker history electrs-app                                                                 
IMAGE          CREATED              CREATED BY                                      SIZE
...
7626a2efbb63   About a minute ago   /bin/sh -c #(nop) COPY --chown=user:userdir:…   393MB
bfbfc289f1a8   About a minute ago   /bin/sh -c groupadd -r user     && adduser -…   328kB    

2. This commit is only a slight improvement in that docker build caches will not get invalidated by e.g. changing Dockerfile or LICENSE, as they shouldn't be, as they don't affect the build/compilation.

Possibly the same could be done for other directories, like doc, internal, but I'm not familiar enough with the code to be confident to make that change. Ideally, the COPY . . statement in the Dockerfile should be rewritten to only copy the necessary minimum to get the build done correctly.

3. I assume that the only build artifact needed for the final image is the electrs executable, dropping these:

root@e84f0882e5fb:/build/target/release# du -sb *
114651560       build
278254888       deps
522     electrs.d
2       examples
2       incremental
504     libelectrs.d

This further reduces the docker image size to 88.8MB

I haven't done much testing on the functionality though. All I can say is that it starts:

$ docker run electrs-app -v -v
Config { log: StdErrLog { verbosity: Info, quiet: false, show_level: true, timestamp: Off, modules: [], writer: "stderr", color_choice: Never }, network_type: Bitcoin, db_path: "./db/mainnet", daemon_dir: "/home/user/.bitcoin", blocks_dir: "/home/user/.bitcoin/blocks", daemon_rpc_addr: V4(127.0.0.1:8332), electrum_rpc_addr: V4(127.0.0.1:50001), monitoring_addr: V4(127.0.0.1:4224), jsonrpc_import: false, index_batch_size: 10, bulk_index_threads: 16, tx_cache_size: 10485760, txid_limit: 100, server_banner: "Welcome to electrs 0.8.9 (Electrum Rust Server)!", blocktxids_cache_size: 10485760 }
WARN - failed to connect daemon at 127.0.0.1:8332: Connection refused (os error 111)
WARN - failed to connect daemon at 127.0.0.1:8332: Connection refused (os error 111)

[Kixunil]

LGTM, if you've seen my brainfart, sorry about that.

[romanz]

Thanks!