Finish building Postgres support (#133)

This includes:
 - Building a Docker image for Postgres as well as SQLite
 - Fuller instructions for usage of the package, including the Postgres builds.

A few related things changed here:
 - `.env` is not used anymore -- the defaults in the Dockerfiles are
   sufficient
 - The Rust version in the Dockerfiles is increased to match the MSRV,
   and with it the Alpine version bumped to one built with that Rust
   version.
 - Cargo dependencies on native-tls and openssl updated to include only
   the `vendored` feature, so as not to require a system openssl
   installation.
 - Two GitHub jobs are set up, to build the two different Docker images
 - The documentation incorrectly suggested using `DELETE .. CASCADE` to
   delete clients. This syntax does not exist, as the cascading delete
   is configured in the schema.

Dockerfile-sqlite

@@ -0,0 +1,26 @@
+# Versions must be major.minor
+# Default versions are as below
+ARG RUST_VERSION=1.85
+ARG ALPINE_VERSION=3.20
+
+FROM docker.io/rust:${RUST_VERSION}-alpine${ALPINE_VERSION} AS builder
+RUN apk -U add libc-dev
+COPY Cargo.lock Cargo.toml /data/
+COPY core /data/core/
+COPY server /data/server/
+COPY postgres /data/postgres/
+COPY sqlite /data/sqlite/
+RUN cd /data && \
+  cargo build --release --bin taskchampion-sync-server
+
+FROM docker.io/alpine:${ALPINE_VERSION}
+COPY --from=builder /data/target/release/taskchampion-sync-server /bin
+RUN apk add --no-cache su-exec && \
+  adduser -u 1092 -S -D -H -h /var/lib/taskchampion-sync-server -s /sbin/nologin -G users \
+  -g taskchampion taskchampion && \
+  install -d -m1755 -o1092 -g1092 "/var/lib/taskchampion-sync-server"
+EXPOSE 8080
+VOLUME /var/lib/taskchampion-sync-server/data
+COPY entrypoint-sqlite.sh /bin/entrypoint.sh
+ENTRYPOINT [ "/bin/entrypoint.sh" ]
+CMD [ "/bin/taskchampion-sync-server" ]