Add make publish: build the image on the Docker host and push it to the registry

script/publish runs script/docker with DOCKER_HOST set to the Docker
host (ssh://root@fsn1app1 unless set) and pushes the image to
git.eeqj.de/sneak/smallwebwaf under four tags: the UTC date as
YYYY-MM-DD, the short commit hash, the branch name and latest. It
refuses a working tree with uncommitted changes, since the hash tag
names a commit.

script/publish

@@ -0,0 +1,51 @@
+#!/bin/sh
+# script/publish: build the image on the Docker host and push it to the
+# registry under four tags: the UTC date (YYYY-MM-DD), the short commit
+# hash, the branch name and latest. The build is script/docker's, so the
+# tests and the linter run first. The host and the registry name are
+# environment variables with defaults, for running it against another.
+#
+# The working tree must be clean: the hash tag names a commit, and an
+# image built from uncommitted changes would carry a hash that does not
+# describe it.
+set -eu
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
+
+: "${DOCKER_HOST:=ssh://root@fsn1app1}"
+: "${PUBLISH_IMAGE:=git.eeqj.de/sneak/smallwebwaf}"
+export DOCKER_HOST
+
+main() {
+    cd "$ROOT"
+
+    if [ -n "$(git status --porcelain)" ]; then
+        echo "script/publish: the working tree is not clean; commit or stash first" >&2
+        exit 1
+    fi
+
+    date="$(date -u +%Y-%m-%d)"
+    hash="$(git rev-parse --short HEAD)"
+    branch="$(git rev-parse --abbrev-ref HEAD)"
+    if [ "$branch" = "HEAD" ]; then
+        echo "script/publish: detached HEAD; check out a branch first" >&2
+        exit 1
+    fi
+    # A tag is [A-Za-z0-9_.-]; a branch name such as feature/x is not.
+    branch="$(printf '%s' "$branch" | tr -c 'A-Za-z0-9_.-' '-')"
+
+    local_tag="$("$SCRIPT_DIR/projectname")"
+    "$SCRIPT_DIR/docker"
+
+    for tag in "$date" "$hash" "$branch" latest; do
+        docker tag "$local_tag" "$PUBLISH_IMAGE:$tag"
+    done
+    for tag in "$date" "$hash" "$branch" latest; do
+        docker push "$PUBLISH_IMAGE:$tag"
+    done
+
+    echo "pushed $PUBLISH_IMAGE with tags: $date $hash $branch latest"
+}
+
+main "$@"