Improve Dockerfile:

* No Cargo.lock or cruft in build directory
* More default Cargo features with overriding option
* Strip executable

Dockerfile

@@ -6,14 +6,16 @@ RUN apk add --no-cache musl-dev pkgconfig openssl-dev
 WORKDIR /src/websocat
 
 COPY Cargo.toml Cargo.toml
+ARG CARGO_OPTS="--features=workaround1,seqpacket,prometheus_peer,prometheus/process,crypto_peer"
 
 RUN mkdir src/ &&\
     echo "fn main() {println!(\"if you see this, the build broke\")}" > src/main.rs && \
-    cargo build --release --target=x86_64-unknown-linux-musl && \
+    cargo build --release --target=x86_64-unknown-linux-musl $CARGO_OPTS && \
     rm -f target/x86_64-unknown-linux-musl/release/deps/websocat*
 
-COPY . .
-RUN cargo build --release --target=x86_64-unknown-linux-musl
+COPY src src
+RUN cargo build --release --target=x86_64-unknown-linux-musl $CARGO_OPTS && \
+    strip target/x86_64-unknown-linux-musl/release/websocat
 
 # Final stage
 FROM alpine:3.15